Erika watched as her sister took off the boy's pants, to her surprise all he had at first was a slit where a boy's penis should be. Her sister started kissing it and rubbing it until something protruded out of it. But it wasn't a penis, it was strap as a dagger, with a little hold at the tip, and made of solid bone. It was like some horrifying combination of a penis, a syringe, and a serpent's fang.
Her sister started licking and rubbing her lover's strange protrusion. Erika was terrified to see what this might lead to but couldn't look away.
Her sister then lied down on her back and spread her legs, presenting the nothingness where her vagina once was to him. She spoke passionately to him, "I'm tired of sucking you off like a schoolgirl. Penetrate me!"
Erika was afraid what that might mean in this odd new world.
The boy seemed nervous, "What if your parents see the blood, or what if I put an egg in you?"
She grabbed him by the collar bone, "I'm a woman now, who cares if someone finds out. Impregnate me, give me a thousand larva!"
He thrust his bone spike into her crotch. The skin broke and the spike went in deep. There was blood everywhere but neither of them seemed to mind. Even though she was being stabbed her sister seemed to enjoy it, moaning with pleasure and rubbing blood on herself.