That is creepy. It's also wrong, abnormal, abusive and amounts to sexual harassment. Don't be confused, what he is doing is overtly sexual and inappropriate, there's nothing nonsexual about it.

You need to tell someone of authority immediately. Tell your mother, no matter how hard that seems. Counseling is free to students at universities, so you could speak with a therapist there who could have real concrete steps for you to take. If there isn't an immediate intervention on your mother's part you should report it to police.

You need to get him out of your household or get yourself out of the household. His inappropriate behavior isn't lessening and could escalate.

I grew up in a similar situation. Don't make the mistake of thinking this is normal. Appropriate dads and stepdads don't do this. They have appropriate boundaries and would never sexualize a daughter or stepdaughter. You shouldn't have this disgusted feeling at the pit of your stomach. Girls in normal homes don't have that creepy, disturbed feeling about their father or stepfather - just people like you and me. It took me a long long time to learn that.