If he was really interested in you, would he still be going after your step-sister? :|
Wouldn't he be putting all his efforts into trying to begin a relationship with you?

Meh, it's totally up to you hun, but if he dates your step-sister, you'd more than likely still have feelings for him... Now if anything, I think that would be pretty damn awkward for everyone and could cause problems.

You are not even dating him and already you say you don't trust him.
Please explain why you would even want to begin a relationship with him when ALREADY you don't trust him.

Maybe speak to you step-sister and let her know the whole things bothering you, and you like him. I'm sure that she'll tell him herself to back off.

Even still though, in my opinion, it's not worth the hassle hun.

Get with someone who you trust as well as having feelings for, and preferably someone who doesn't flirt with a family member.