Author's Notes: Written for my lovely friend Kat Ducat, because she is wonderful, and because the world always needs a little more Blackcest.

Warnings: Incest (obviously!), underage (they're both in school), bondage, object insertion, and just a dash of exhibitionism.

Enjoy!

)O(

Narcissa was walking to class – perfectly innocently, with absolutely no thoughts that were anything less than entirely chaste in her mind – when she felt a pair of arms snake around her middle and a pair of breasts push against her back.

"So glad I caught you, Cissy," Bellatrix breathed in her ear. Her hand moved to tweak one of Narcissa's breasts and Narcissa flushed.

"Not here, Bella- not now," she muttered, but Bellatrix's finger was circling her nipple so gently, so delicately, and Narcissa could already feel it rising into a hard point. She pressed her lips together, annoyed at herself for being so easily manipulated.

"Eager, hmm, Cissy?" Bellatrix's hand slipped down between Narcissa's legs and she stroked her through her skirt. Narcissa thrust automatically against her sister's hand while her face burned, knowing that, at any minute, someone might come around the corner and see them.

"I said… not now…" she mumbled half-heartedly.

Bellatrix paid her no mind.

"Come here," she whispered, and the next thing Narcissa knew, she was being dragged into an empty classroom. Bellatrix pushed her down into one of the desks, then shut the door behind her and turned on her little sister with a predatory smile. A shot of pleasure ran between Narcissa's thighs and she bit down on her lips and watched as Bellatrix leaned against the door to unbuttoning her blouse.

"I've been thinking about you all day, Cissy," she murmured, dropping it to the floor. Her bra followed quickly, and Narcissa reached out for her, wanting to feel her breasts in her hands and hear her moaning in her ear, feel her hot breath against her neck…

"Ah-ah." Bellatrix's lips curled and she drew her wand and flicked it at Narcissa, who felt her arms being wrenched behind her back. When she struggled, she found that she couldn't move them.

"I can't have you flailing around while I play with you, Cissy, darling," Bellatrix murmured. Narcissa tried to move again, but her arms were stuck behind the back of her seat as if they had been tied there. Bellatrix perched on the desk with one leg on either side of her and she hummed softly while she unbuttoned Narcissa's blouse and stroked her stiff nipples through her bra. Narcissa's head fell back and she moaned with every caress that Bellatrix gave her pert little breasts, and when Bellatrix's hands left them, she let out a small cry of protest and looked up at her, only to see Bellatrix smirking and sinking to the floor between her legs, under the desk.

"I was thinking of you during transfiguration," Bellatrix murmured. Narcissa clamped her thighs together on her head, but Bellatrix grabbed them and forced them back apart. She was so much stronger. "Every time I touched my wand, I thought of you… of how pretty you'd look with it shoved up into your tight little body… it's twelve and three quarter inches, Cissy," she added sweetly. "You'd break in two if you tried to ride a man with a cock so big – but I'm sure that you could take my wand."

Narcissa turned red at the very idea and bit her lip, imagining the pain of having so well-endowed a man inside her, and then the pleasure – pleasure still laced with pain – of having Bellatrix fuck her with her wand.

"Let's try it, shall we?" breathed Bellatrix, raising her wand and gently massaging Narcissa's clit through her knickers with the handle. Narcissa's legs trembled and she moaned and bit her tongue, squirming while the handle pressed hard against her stiff clit.

"You're drenched, Cissy." She used her wand to move her knickers aside. Narcissa's pussy was so wet and swollen, and the air felt pleasantly cool against it. Bellatrix pressed the handle of her wand – so thick and solid – against Narcissa's opening, then smirked up at her, gently working the handle back and forth against her hole. "I think I'd like to hear you beg for it. Moan for it."

"I- I–" Narcissa stammered breathlessly. "I want your wand i- inside my- my- oh, don't make me say it…"

"Your pussy," Bellatrix finished for her, and then she rammed her wand deep into Narcissa and giggled as she writhed on it. "There, you like that, don't you, darling? It feels good, doesn't it?"

"Yes!" Narcissa lifted her legs and propped her knees against the edge of the desk to give better access. Bellatrix's head was between her thighs in a second and Narcissa groaned as she felt her knickers slip to the side and her sister's tongue begin to work against her clit. She arched and pulled as hard as she could against the invisible bonds around her wrists, desperately wanting to tangle her fingers in her sister's hair and hold her against her. Bellatrix moaned quietly and Narcissa's toes curled, for she loved nothing better than feeling as if she had aroused her sister.

Bellatrix gripped Narcissa's thigh with one hand and held it in place while she used her other hand to pump the wand into her. Every hard stroke felt like she was splitting Narcissa apart, and she loved it, loved the pain that she could endure for her sister, loved how powerless she felt with her legs apart and her most sensitive parts being used so roughly…

Bellatrix sucked hard on her clit and Narcissa couldn't quite stifle a cry of pleasure. Her whole body tightened and a wave of sharp, intense pleasure washed over her. Bellatrix was still pounding her wand in, and it was all too much.

"S- stop- stop it, Bella–" Narcissa whimpered, heaving for breath, and Bellatrix looked up at her with a cheeky smile.

"What would you do to make me stop?" she breathed.

"A- anything- anything!"

Bellatrix slowed her pace, then finally slipped the wand handle out of her. It was glistening with her juices, which Bellatrix licked off idly, looking up at her sister with a thoughtful expression.

"Then return the favour for me, darling," she murmured. "And open the door. I want people to see you on your knees for me."

)O(

Fin