It's bad to have sexual relations with a step sibling.
James' step sister is about his age and very attractive.
James wants to have sex with someone attractive.
James asks his step-sister if any of her attractive friends are currently single.
James invites his stepsister out drinking, and they end up sleeping together.
James' step-sister hooks him up with one of her friends' cousins.
When James' mother and stepfather find out, they're extremely upset and tell the two step-siblings to stay away from each other.