Making this as short as possible. I understand what you feel. My advice is.. Go and find someone better. If you dont trust him already.. dont go out with him. Move on and forget about him. If you were to go out with him, and you dont trust him, the relationship would be unstable. As for your step sister and him, it would be awkward seeing them. So try and convince your step sister to listen to you and not go out with him.
Hope this helps... - Lana <3