You're having anxiety about sex, and feeling sexual urges, which is perfectly normal.  You are afraid it will be painful.
Your brother is a male you've know your whole life, and so he seems safe to you...that he would never intentionally hurt you. You may be wanting your future husband to have the same attributes as your brother.

Relax. Stand by your convictions, and know that when the time comes, things will go as they should and everything will be fine.

Hope that helps, good luck!