Author's Notes: Written for Round One of the Pole Vault event (writing about your OT3, begins with a 500-word drabble) in the 2012 Hogwarts Games on the HPFC forum, with the optional prompt "jealousy"
Warnings: Incest and implications of underage sex (nothing graphic).
"Isn't she pretty?" Bellatrix murmured in Andromeda's ear. Her arms were looped around Andromeda's waist, holding her gently against her as the two of them looked down upon Narcissa. She was curled in bed, knees against her chest and the blankets all twisted about her, with her pale hair fanned out on the pillow behind her. "She's growing up so… attractively…"
"You can't be serious…" Andromeda twisted a little in her sister's arms, looking at her with an expression of incredulity. "You can't… be thinking…"
"Thinking what, darling?" Bellatrix breathed.
Andromeda felt her throat tighten automatically, and she pulled away from Bellatrix. "You don't- want her… do you?"
She shouldn't have even bothered to ask. The answer was obvious in the way Bellatrix smirked, in the way her breath was speeding, her breast heaving and her eyes shining.
"Of course I want her."
"What? Are you going to tell me that I shouldn't want her?" Bellatrix asked derisively. "And why? Because she's our sister? Because she's not of age yet? That didn't stop me with you…" Her voice went soft, low and sensual and velvety, the tone of voice that she knew sent shivers up Andromeda's spine and made her want to melt into her arms. "So why should it stop me with Cissy?"
"That's not what I was going to say," Andromeda told her quietly. "That isn't what I meant."
Andromeda's lip trembled. She tried to stop it, tried to keep her face impassive and not show how much it hurt, but finally, she burst out, "Because you're supposed to only want me… only love me!"
Bellatrix looked rather startled by this pronouncement. She opened and closed her mouth several times, as though searching for words and unable to find them, then, at last, she managed to say, "Well- of course I love you, Andi."
"If you loved me, then you wouldn't want her!" Andromeda hissed, pointing at her younger sister. It stung her, right to her heart, that Bellatrix could desire anyone else – though perhaps she shouldn't have been surprised. Everyone wanted Cissy, everyone had always wanted Cissy far more than they would ever want Andromeda – why should Bellatrix have been any different?
"Don't be so childish, Andi," Bellatrix told her. Her momentary muteness had disappeared, and now she simply appeared irritated by her sister's behaviour. "What, am I only allowed to love one person at a time, now?"
"Yes!" Andromeda cried, stamping her foot.
The word came out louder than she had expected it to. Narcissa's eyes flew open and she sat bolt upright in bed, looking at her sisters with an expression of profound confusion.
"Bella? Andi?" she asked, rather timidly.
Andromeda fell silent, and so did Bellatrix, who looked slowly back and forth between her two sisters for several long moments before she spoke.
"If it will make you feel any better, Andi," she said, "I suppose that we can… share her."
"You know," Bellatrix breathed. "The three of us."