Yeah, I understand the attraction and fantasy. However, I would have to agree with DB Lady. You need to find someone other than your step brother's to play with. PLEASE BE SAFE though. There are a lot of crazy peeps out there that do not care if they cause harm to you.
Also, if you have someone you can talk to about your feelings that you trust, I encourage that as well. I agree that it will complicate so many things for you long term if you continue having relations with them.
Good luck buddy... Best Regards!!
P.S. Don't think badly of yourself, just stick to your guns, and move on! :)