Why not rape the both. Rape and brutalize the older one until she gives you her younger sister, tie the older sister up. Have some fun with the youngest one, then you and the older sister tag team the younger sister. Make baby sister eat your cum out of her big sisters ass while you fuck big sister's pussy. Whisper to the big sister that she either fucks her little sister with her foot or you're going to fuck the little sister with a bat.
When it's all said and done, tell the little sister that her older sister pretty much fed her to you, then participate as the younger sister takes her revenge.