Just sat in the dining room alone with your step dad, he doesn't like you that much and starts a bit of early morning banter "So Incey, dont know if your heard us going at it last night, your mums great in bed" "So is your daughter"
Imagine a step sister being like noo... wtf you weird cunt, and then things never ever being the same again.
Then you'd just do her friends, even the ugly ones, and eventually once word got around...she'd get jelly and have to give it a go.