If you don't feel comfortable you need to remove yourself form the situation. I understand that he makes you uncomfortable. Thats a shitty situation.

I realize this is something that may be hard to discuss openly, but if by "tried" you mean he has straight up attempted this, then the story needs to stop there. Whatever it takes to ensure your safety needs to be done. If your mom explains it away as "hormones" you can clearly see that even she realizes its sexual and still defended him. Your mom is of no help here anymore. I'm sorry. Thats just the straight up truth. I'm going to be very blunt here. Your mom, unfortunately seems willing to look past your concerns and safety to not lose her relationship with him. Don't catch a rape in an attempt to not ruin your relationship with your mom.

You never mentioned your dad past the separation and unfaithfulness. If you have a relationship with him I would start there. Honestly, even if you don't I'd still start there. As others have stated you need to find out if he has a legal history of this. If he does this may be an issue that is quickly resolved. If he doesn't, realize that the solution may be on you. In a perfect world you can make your mom understand, but nothing about this world is perfect.

I browse here off and on but rarely contribute. Its probably because of my Y. This stood out though. I understand what its like to have a step parent cloud your parents judgement to the point of endangering yourself and your relationship with them. We have very different situations, but that pain is universal. Its hard to feel like you can turn to somebody, when the first person you'd turn to is the last person that can help you. I won't pretend that it'll all be fine in the end. I wish I could have the relationship with my father that my other siblings have, but its always going to be strained. In the end I wouldn't change anything. At some point you have to put you first.