Her boyfriend never fucked her pussy, only her little sister’s. She asked him why once and he explained that her little sister’s pussy was almost as tight as her ass. It was understandable that  her boyfriend only fucked her in the ass, while letting her little sister take his cock in her pussy, since it was obviously true that her 18 year old sister was tighter. As long as he let her eat his cum back out of her sister’s tight little cunt, she figured she was fine with things the way they were.