I'm going to make out with my friend's step-brother
How did you come to the decision to make out with him?
I think he likes me, so I figure "why not?"
If you like him, it cannot be helped but you still have to be mindful that he is related to your friend. It may seem like betrayal to your friend for you to go for her relatives. Is your relationship worth this man?
Now that you say it like that it does sound like a bad idea. I'm going to call him and end our arrangement before things get out of hand.
Thats definitely the safer bet. If you really don't want to do that you can talk to your friend about it first but what you're doing is a very good way to avoid the potential problems.