I would try to live with your aunt.... And by the way, I'm having a real hard time trying to understand how someone "unintentionally" sexually abuses someone, or even just abuses someone..... At some point responsibility must enter the picture.
And I mean responsibility for he who abused you, not you yourself.... Don't blame yourself at all please.
Yes, I also agree with the "anonymous" comment. I've never heard of "someone 'unintentionally' sexually" abusing someone. It sounds more like that your step dad started showing you some affection (i.e. Hugging & kissing in an acceptable parental way) and then got carried away, i.e. Went TOO FAR, and sexually abused you. In that, he is still VERY MUCH responsible. He violated your trust, and will take quite a long time for him to earn that trust back. BUT, You did nothing wrong, and you should be angry with him. Does your mom know what he did? If not you should tell her, AND if she doesn't believe you, or thinks he did nothing wrong, then you need to tell some other responsible adult, like your school-counselor/social-worker.
As far as you feeling really uncomfortable around him, this is very normal. Your probably feeling mixed feelings around him. Confusing mixed feelings. Your uncomfortable around him, maybe even angry with him...but you may still love him. This is also very normal. When my mom sexually abused me, I hated her, but I loved her, but I still hated her..hate - love, hate - love. Like I said, very confusing.
Yes, you should go live with some other relative, like your aunt.
For 2 reasons: 1) like you said, you are very uncomfortable around him,
BUT 2) if he sexually abused you once, he may do it again.
Hope something I said (above) helps.
Respectfully yours, Heathland