"You have to swear it," Lily insists, holding out her hand. It's pricked in the palm, a cat's scratch worth of blood smeared along the surface, and Petunia looks at it as though it might bite. For all she knows, it could, after all - her blood and Lily's aren't the same anymore, perhaps never were.
"Tuney!" Lily calls, sharply.
"It won't work," Petunia says, but her hand moves of its own accord.
"It's got to, it's magic, what else is it good for?" Lily grabs her sister's hand, and Petunia's hairs all stand on end at once. She'd swear the light changes, that their bedroom walls are washed in a sudden electric energy, but looking again she can see it's all the same yet. Wishful thinking, then.
"What now?" she asks. She wishes it were over already. She wishes it had never begun.
"I swear - on my magic," says Lily, as if by sudden inspiration, "that this will never come between us. We're sisters! We share everything. And even if we fight about it sometimes - because sisters share fights too -"
Petunia's smile is as wry as any twelve-year-old's can be.
"Anyway this can't come between us. If I'm magic she has to be too! And I'll find a way to make that happen or I'll die trying," Lily says, and Petunia winces. Nothing else happens - no magical shock, no thrumming, no sound or sight or even stillness to indicate anything has changed. Petunia just barely refrains from rolling her eyes; it doesn't seem like it would be proper.
After a moment, Lily offers Tuney's hand a little squeeze before releasing her.
"I didn't swear to anything, though," Petunia says after a moment.
"You didn't have to say it out loud," Lily says. "The magic knew."
Petunia is more disturbed than comforted by this, but then Lily points out that they really ought to go wash their hands, and Petunia remembers the blood on them - Lily's blood, and her own, now smashed together and maybe doing something as wild and freakish as making her magical too - and swiftly agrees.
She doesn't think of this moment again for another ten years, when she irritably snaps "Light already!" at a nearly burnt-out candle in a jack o' lantern and, to her great surprise, it does - and glows green.