You wake up early one morning while on a family vacation and find your little sister in the pool wearing your girlfriend's bikini. She taunts you because of how TINY your girlfriend's boobs are. Don't you like huge boobs? I guess your girlfriend has a big... personality? to make up for her lack of tits. You tell her that you're going to tell your mom, dad, and girlfriend but offer her a way out of getting in trouble. This isn't the first time you've blackmailed your sister into sucking your dick or fucking her tits so that she doesn't get in trouble and it definitely won't be the last. You convince her to let you cum on her tits because she can just wash it off, right?