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Sirius Black may have tortured Severus Snape but Severus banged his sister so who's really winning?
Stats:
Published:
2026-03-05
Completed:
2026-04-06
Words:
102,216
Chapters:
20/20
Comments:
27
Kudos:
137
Bookmarks:
32
Hits:
4,287

Just For Once

Summary:

The end is near, Dumbledore has one last person he can call upon to help, though he had hoped to never have to.
Rose Black - Sirius Blacks twin sister - has been in hiding for over a decade, preparing herself for the day Albus would call for her, and she will do whatever is necessary to protect the children of Hogwarts.
Sirius Black hasn't seen his sister since he left for Hogwarts at 11 years old - he thought she was dead, or worse, but no. Now he's struggling to understand the person his dear sister has become.
Severus Snape has enough going on in this war, the last thing he needs is another member of the Black family in his way - problem is, he doesn't hate this one like he does her brother, in fact he's struggling to describe what it is that he feels for her.

AKA: Sirius has a sister, she's the DADA professor now, Snape is kind of obsessed with her, Sirius is struggling to get her to connect, and she's just trying to teach the students to fight.

- Screw JK Rowling, Trans lives are human lives and trans women are real women and they're all hot. -
- Also screw AI, save the planet

Notes:

Bare with me here, I have a vision.
UPDATED CHAPTER: This chapter has been updated and is much longer and more details so please re-read it before continuing!

Chapter 1: Hello, brother.

Chapter Text

The cold breeze flew through her feathers as she soared through the fog toward the mountains. The distant flap of Corbière’s wings could be heard at her side, and she could tell he was slowing his pace to match hers. He always was faster; She guessed being born a Raven had its perks. Rose tucked her wings in close to her chest, allowing her to swoop down below the thick air, and the familiar peaks of the castle came into view.

She landed not far from the gate, and as her talons hit the soft grass she transformed back into herself. Corbière landed perfectly on his perch atop her staff and ruffled his feathers after the long flight. The castle looked a lot more daunting up close. She took a steadying breath before stepping toward the gate, which opened as if expecting her, though she supposed it probably was.


“I’m not saying this new girl isn’t experienced, I’m just saying that bringing in another new professor is a risk here, Albus, and unnecessary when we have the teachers we need already.” Severus paced around the headmaster’s office. The concept of a new variable in this already manically complicated situation was giving him a very bad feeling.

“For once, and I can’t believe I’m saying this, I agree with Snape.” Sirius spoke half-heartedly from the lounge chair and barely lifted his head from where it rested against his arm. Snape’s head snapped toward him in slight shock. He almost wanted to endorse the new professor purely out of principle. “We don’t know that we can trust her, and if we can, then we have another person to protect. It’s a liability.” Sirius was very blatantly avoiding Snape’s gaze, appearing very interested in the flames dancing in the fireplace.

“We can trust her. You have my word. But either way, I did not bring the two of you here at such an inconvenient hour for your opinions on the matter. I brought you here because, well…” Albus trailed off, looking between the two equally grumpy, fed-up professors. “She is not your average witch, and an even less average professor.” Dumbledore drifted toward the window overlooking the courtyard as he spoke. Snape could tell he had more going on than he was explaining, and Snape would almost certainly disagree with his plans, which was probably why Albus hadn’t told him.

“What exactly do you mean?” Sirius began to sit up. He was clearly getting intrigued by this mystery woman. Good god, Severus could not watch him fawn over yet another woman.

“Perhaps it may be easier if you two meet her. She’s just outside,” Albus said as he turned toward the entryway to his chamber, which was now slowly opening itself.

Standing in the entryway was a rather elegantly dressed woman with long black hair. She had multiple skirts layering and flowing down to the floor and a loose white blouse cinched with a black vest. And to her left was a staff made of wound and warped wood. At the top sat a carved wooden raven with glass eyes embedded.

The whole sight was truly something. She was so elegantly casual in her presence, but the magical energy that came off her in waves almost brought a tear to Snape’s eye. He had never been so entirely astounded by a sight until now. He glanced at Sirius, and he appeared to be in a similar state. However, his brows were furrowed and he looked almost - no, definitely - furious.

She looked into Sirius’s eyes and stepped into the office. “Hello, brother.”

That singular word halted everything in Snape's mind. Every whirling thought he had been experiencing came to a very sudden and crashing stop as that one word was said. Sirius didn’t have a sister. He didn’t have any family. This had to be some sort of joke.

“Rose.” Black replied. It was clearly meant to be said as normally as he could manage; however, the questioning tone was unmistakable, and Severus saw something in Black that he hadn’t seen in a long time. He was shaken, afraid.

The woman - Rose, he presumed - let her hand drop from where it held the staff. It remained perfectly standing without her grip as she sauntered toward Snape, standing an appropriate distance from him and extending a hand. Sirius’s eyes were burning a hole through Severus, and he now couldn’t resist taking part in the game presented to him.

He took her outstretched hand and brought it to his lips, giving it a slow and deliberate kiss before rubbing his thumb over the back of her wrist. “Severus Snape, Potions Master.” He maintained eye contact. She had completely black eyes, no differentiation between iris and pupil, just black.

She smiled widely at him. Good lord, that smile. He might fall over.

“Rose Black, Defence Against the Dark Arts. It is an absolute pleasure to meet you.”

She was playing the game too, and he could see Sirius fidgeting behind her. He was furious. She eventually withdrew her hand from his, and he felt that loss of contact deep inside himself.

Rose turned her attention toward Albus and bowed her head slightly. “Headmaster, thank you for giving me this opportunity.”

He smiled widely before chuckling and reaching out to hug the girl. “Oh my darling, drop the formality. I have missed you terribly!”

She giggled as he threw his arms around her, and that laughter made Snape’s breath catch in his throat. He looked away and caught Sirius’s gaze, throwing him a slightly questioning look that he hoped conveyed what he was wondering:

Good god, you have a sister? And she’s beautiful? And friends with Albus? And she’s the new professor? How did we not know you had a sister?

Or something to that effect.

Sirius clearly caught at least half of the message because he looked almost ashamed and pulled his gaze from Severus to the ground.

Severus simply had to know more about this situation.

Sirius watched the embrace between Rose and Dumbledore with thinly veiled irritation. The moment they separated, he pushed himself up from the lounge chair, running a hand through his dark hair in agitation.

“Alright,” he said sharply, pacing a few steps forward. “That’s enough of that.”

Rose turned to face him fully, her expression calm in a way that only seemed to make him more irritated.

“You,” Sirius continued, pointing at her as though he feared she might disappear if he didn’t keep track of her. “You vanish for - what, a decade? More? No letters, no explanations, not even a rumour of where you went. And now you stroll into Hogwarts like nothing happened?”

Snape folded his arms, watching the exchange with quiet fascination.

Rose tilted her head slightly. “Hello to you too, Sirius.”

“Don’t” he snapped. “Don’t do that calm voice thing with me. I want to know why you’re here.”

Dumbledore quietly stepped aside toward his desk, allowing the confrontation to unfold without interference.

Rose moved back to rest one hand lightly on her staff. The carved raven at its top seemed almost to stare directly at Sirius.

“I’m here because Albus asked me to come,” she said simply.

Sirius barked a humourless laugh. “That’s not an answer.”

“It is,” she replied. “Just not the one you want.”

Snape’s gaze flicked between them. The resemblance was subtle but undeniable now that Sirius stood closer: the same sharp cheekbones, the same intensity behind their eyes, though hers were unsettlingly dark.

“So, you just decided to come back?” Sirius pressed. “After all these years?”

Rose’s smile faded slightly. “I didn’t decide anything,” she said. “The war is escalating. The students here are in danger whether the school admits it or not.”

Snape’s eyes narrowed slightly at that.

“They need more than theory,” she continued, her voice steady. “More than textbook curses and polite duelling etiquette. They need to know how to defend themselves when someone actually intends to hurt them.”

Sirius crossed his arms. “And you’re the expert on that?”

“Yes,” she said without hesitation, the certainty in her voice settled into the room with a crushing weight.

Snape felt a faint shiver of interest. Arrogant answers usually irritated him, but hers carried none of the empty boasting he was used to hearing from mediocre duellists. She sounded like someone who knew her worth, not out of pride - out of experience.

Sirius studied her face; suspicion etched into every line.

“You expect me to believe you came back out of the goodness of your heart?” he asked.

Rose shrugged lightly. “Not exactly.” She began pacing toward the window, surveying her surroundings - etching every detail into her mind.

That answer made Sirius’s eyes narrow.

“Then what?”

She glanced briefly toward the window overlooking the dark courtyard, her expression becoming thoughtful.

“I’ve spent years studying magic that most people pretend doesn’t exist,” she said. “Magic used in real fights, real wars. Magic people like Voldemort aren’t afraid to use.”

Snape stiffened slightly at the name, though he said nothing.

“And Hogwarts,” she continued, looking back at Sirius, “is still teaching children as if the worst thing they’ll face is a poorly cast jinx in a corridor.”

A faint smirk tugged at Snape’s mouth despite himself - She wasn’t entirely wrong.

“So, you’re here to fix that?” Sirius asked sceptically.

Rose nodded once. “I’m here to teach them how to survive.”

Silence filled the office for a moment.

The fire crackled softly.

Snape finally spoke.

“Survival,” he said slowly, “is a rather ambitious subject for a school curriculum.”

Rose turned toward him again, that sharp smile returning.

“Only if the teacher isn’t capable of it.”

Sirius groaned and dragged a hand down his face.

“Oh brilliant,” he muttered. “Now there’s two of them.”

Dumbledore chuckled quietly behind his desk. “I did warn you,” he said mildly. “Miss Black is… somewhat unconventional.”

Rose glanced back at Sirius, her expression softening just a fraction.

“And for the record,” she added, “I didn’t come back because it was easy.”

Sirius’s jaw tightened. “Then why?”

For the first time since entering the room, Rose hesitated.

“Because” she said quietly, “whether you like it or not, we are still family.”

The words hung between them - Sirius looked away first, unable to withstand the look of expectancy in his sisters’ eyes.

Dumbledore watched the tension settle over the room for another moment before clapping his hands together lightly, the sound bright against the quiet crackle of the fire. “Well then,” he said pleasantly, as though they had just concluded a perfectly ordinary staff meeting rather than a tense family reunion. “Now that introductions have been made, we should return to the matter of hospitality.”

Sirius muttered something under his breath and dropped back into the lounge chair. Dumbledore turned toward Rose with a warm smile. “You must be exhausted after your journey, my dear.”

Rose inclined her head slightly. “A little.”

“Then it would be best if you had some time to settle before dinner.” Dumbledore’s blue eyes twinkled as he looked toward the Potions Master. “Severus, if you would be so kind as to escort Professor Black to her chambers.”

Snape stiffened almost imperceptibly - Of course.

He could practically feel Sirius’s eyes burning into the side of his head. Maybe this would be fun, “Certainly, Headmaster,” Snape said smoothly.

Rose lifted her staff, the raven at its top giving a faint creak as it settled into her grip. She gave Dumbledore a small nod before turning toward the door.

Snape moved ahead of her, robes sweeping behind him as he stepped into the corridor outside the office. The door closed quietly behind them, muffling whatever comment Sirius was undoubtedly making inside.

For a few moments they walked in silence.

The corridor was dimly lit by torches along the stone walls, their flames flickering softly in the evening draft. Their footsteps echoed faintly on the flagstones. Snape glanced sideways at her, up close, the strange quality of her eyes was even more unsettling. Entirely black, absorbing the light rather than reflecting it.

“How fortunate for you,” he said coolly, “to arrive at Hogwarts with such… impressive recommendations.”

Rose hummed lightly. “Is that your way of asking how I know the Headmaster?”

“It is my way,” Snape replied, “of asking several questions at once.”

“I suspected it might be.” She smiled faintly at him.

They turned down a spiralling staircase leading toward the upper corridors. Snape clasped his hands behind his back as he walked.

“Professor Black,” he said after a moment, the title sounding deliberate. “You must forgive my curiosity, but your sudden appearance has caused some… confusion among the staff.”

“That’s a polite way of saying no one knows who I am,” she said.

“Precisely.” They kept walking, Severus tried his best to avoid looking in her direction - fearing that if he looked again he may not be able to tear his gaze away.

Rose did not seem offended. “I’ve been away for a long time,” she said simply.

Severus hummed lightly in acknowledgement, it was clear she wasn’t the type to just ramble about her personal history and Severus was attempting to seem his most unbothered and uninterested.

They continued walking, the staircase opening into a quieter corridor lined with tall windows overlooking the dark grounds.

Snape stopped briefly beside one door.

“Your chambers are here. Adjacent to the Defence classroom.”

Rose stepped closer as he opened the door with a flick of his wand. The room beyond was spacious but simple: a large four-poster bed, a desk near the window, shelves already stacked with blank parchment and a few basic texts. A fireplace crackled gently in the corner.

Rose stepped inside, her skirts whispering across the floor.

“It’s lovely,” she said.

Snape remained by the doorway. He should just leave.

“You have not answered my question.”

She allowed her staff to stand by itself beside the desk and turned to face the tall cloaked man.

“Which one?” She smiled innocently, the perfect picture of grace and formality.

“How someone who has been absent from the wizarding world for years is suddenly qualified to teach defensive magic at Hogwarts.” He took a step into the room - he really should just leave.

Rose turned away from him and for a moment the playful ease she had worn earlier slipped, revealing something sharper beneath.

“I never said I was absent from the wizarding world,” she replied.

Snape’s eyes narrowed slightly.

“No?”

“No.” She folded her hands loosely in front of her. “I was simply… elsewhere.”

“Doing what?”

Rose studied him for a moment, as if weighing how much to say.

“Learning.”

Snape raised an eyebrow. “That is a remarkably vague answer for someone entrusted with the safety of Hogwarts students.”

Her smile returned slowly.

“You don’t trust me.”

“Trust -” Snape said calmly, taking another step forward before responding “-is not something I distribute freely.”

Rose nodded once, as though that confirmed something she already suspected.

“Good,” she said.

That answer caught him slightly off guard.

“Good?”

“If you trusted me immediately,” she said, “I’d be disappointed.”

Snape stared at her for a moment, her unwavering smirk was intoxicating. Unexpectedly, the corner of his mouth twitched. Why did that keep happening?

“You may find,” he said dryly, “that Hogwarts is an excellent place for disappointment.”

Rose laughed softly at that. Oh Merlin, that laugh scratches something in his head.

“Something tells me I’ll enjoy working here.”

Snape turned toward the door - “Dinner is in an hour,” he said. “The Great Hall.”

He paused before stepping out.

“And Professor Black?”

“Yes?”

His dark eyes flicked briefly to the strange raven-topped staff standing by the desk, the Raven always seems to be looking at someone, maybe he’s just paranoid.

“Hogwarts has a tendency to reveal things about people,” he said quietly. “Eventually.”

Rose tilted her head slightly - “I’m counting on it.”

Snape gave a short nod and swept out into the corridor, robes trailing behind him. He fought the urge to look back at her, to check if she was watching him leave – why would she be watching him leave? He didn’t want her to watch him leave.