I am so glad you wrote. You may be young but you know, as well as anyone older would, what is inappropriate and threatening behavior. What he is doing is a big deal. It is no laughing matter. Intentional behavior is not a “slip.” His “tickling” is abuse. Being “very sexual” is not an excuse for crossing boundaries with kids.
No kid should feel scared and unsafe in her own home. No kid should be worried about what her stepdad will do next.
Since you don’t quite trust your mother to do what she should, please, please talk to your school guidance counselor or a teacher you trust right away. Ask them to meet with you and your mom to support both of you and to help your mom understand what she needs to do in this situation. Your stepdad needs to leave. Hopefully, knowing what you have been going through will tip the balance so that your mom can decide to get this cheating abusive guy out of the house and out of all or your lives before something worse than “tickling” happens.
Trust your instincts and good sense. This situation isn’t good for anyone.
I wish you well.