Author's Notes: Written for Lady Phoenix Fire Rose's One Hour Challenge/Hardest Challenge Ever with the pairing Bellatrix/Narcissa and the prompt "You are a very bad girl".
Narcissa was in the library of Black Manor, searching for a book and not thinking about her sister at all, when she felt arms wrap around her waist.
"It's been a long time, Cissy," Bellatrix whispered.
Narcissa jumped and inhaled sharply. Just feeling Bellatrix's arms around her was strange and more familiar than it should have been. She felt a chill run down her spine as Bellatrix ran her hands slowly over her flat stomach.
"B- Bella…" Narcissa murmured, pulling away from her a little. "It– hasn't been a long time… I saw you at breakfast–"
"You know what I mean…" Bellatrix pressed closer against her little sister and her breasts swelled slightly against Narcissa's back as she breathed in the scent of her hair. "You know that I mean it's been a long time since we…"
"Don't you dare say it, Bella." Narcissa tensed up automatically and pulled away a little. She looked back over her shoulder at Bellatrix and saw her grinning wickedly. "You know that I don't- I don't want to do this anymore…"
"Don't be silly, Cissy, of course you do." Bellatrix's hand was travelling up Narcissa's torso and she jumped when her fingertips brushed against the underside of her breast. "You say that you don't want to do it because Mother and Father would say it was wrong, but you and I both know perfectly well that it's what they say that's wrong…"
"Let go of me," Narcissa moaned quietly. She knew that she didn't sound even a little bit convincing, and Bellatrix giggled softly, sweetly.
"No," she whispered. Her hand moved up to cup Narcissa's breast and she arched her back a little as Bellatrix pinched her swollen nipples. Her legs weakened and she slumped back into Bellatrix's arms.
"Oh, Bella…" she breathed while her sister squeezed and pinched her breasts. "Oh, no, please…"
"Don't pretend that you don't like it." Bellatrix dug her nails into Narcissa's nipple and she gasped, sure that Bellatrix was cutting it straight through her blouse and bra. She squirmed in pain and Bellatrix let out a low, gravelly moan. She loved her sister's discomfort, Narcissa had known her long enough to understand.
"Oh… Cissy," she whispered, pulling hard on the nipples, then she spun Narcissa around and kissed her hard on her mouth. "You've been denying me for far too long…"
Narcissa couldn't deny that, didn't deny that. She would have been lying if she had tried to tell Bellatrix that she had not missed her every second that she spent without her. She had thought about Bellatrix every night that she lay in bed alone, and her thoughts would eventually become so intense, so erotic and beautiful, that she could not help letting her hand slip between her thighs. But nothing that she could do for herself – nothing that she had ever done for herself – could even come close to matching the sort of pleasure that Bellatrix could give her.
"You've been a very bad girl, Cissy." Bellatrix pulled back and smirked. She grasped Narcissa by her shoulders and pushed her back against the bookshelf. Narcissa felt the spines of the books digging into her shoulders and back, but she didn't care, because Bellatrix's hand had moved to her hip. She stroked it slowly, carefully, and Narcissa moaned encouragingly, because she knew where Bellatrix's hand was going to go next.
"Say what?" Narcissa's mind was hazy, far too focussed on Bellatrix's hand to be able to register what she was telling her to do. Her sister's fingers had reached her knickers now, and one fingertip slid up and down along her slit. "What am I supposed to say, Bella?"
"Say that you're a bad girl." Her fingers slipped beneath the fabric and Narcissa's muscles clenched automatically. Her breath came in short gasps and she bucked against Bellatrix's hand.
"O-oh!" It was too difficult to speak – her mind was empty except for the thought that she needed more of her sister's hand, needed Bellatrix's fingers inside her…
"Say it, Cissy!" Bellatrix grasped her little sister's clit between her thumb and forefinger and pinched it hard. Narcissa let out a cry of mixed pain and pleasure. Her whole body jerked against Bellatrix and tears came to her eyes.
"Bella, that hurts!"
"Say you're a bad girl!" Bellatrix ordered her. "Say it, or I swear, this is the least of what you'll get…"
Narcissa clung to the bookshelf, rocking her hips against Bellatrix's hand. She would never confess to her sister how good it felt when she was hurting her. Narcissa didn't like to think of herself as a masochist; she preferred to continue letting Bellatrix believe – as she presumed that Bellatrix always had – that she, Narcissa, only defied her because she knew that Bellatrix loved to cause her pain.
"Say it, Cissy!" Bellatrix dug her fingers hard into Narcissa's clit and Narcissa let out a small scream. She lifted one leg automatically, bracing it against the bookshelf as she writhed.
"I'm- I've been–" Her cheeks flamed red and her words came out in a jumbled moan. "Averybadgirl…"
"Speak up!" Bellatrix gave her a quick slap across the face, then two across her breasts.
It was all Narcissa could do not to sob in delight. "I've- I've been- I've been a very bad girl to deny you, Bella!"
Bellatrix let out a low moan and she released her grip on Narcissa's clit. Her fingers sank inside her, curling up to touch the most sensitive spot inside her little sister's body. Narcissa moaned and buried her face in Bellatrix's shoulder, allowing the pleasure to wash over her.
"That's right, Cissy," Bellatrix whispered breathlessly. She plunged her fingers in and out and let Narcissa moan and gasp desperately against her. "And you aren't going to deny me anymore, are you?"
Narcissa shook her head, gripping Bellatrix tightly and riding her hand.
How could she deny her sister anything when Bellatrix could put her in a state like this?