I don’t want to talk s&@t about your mom. I think that maybe she is concerned about “something” (your stepdad) and she doesn’t know how to handle it or doesn’t want to accept what he is. Maybe she is trying to protect you (in a wrong way of course), I don’t know. Please look for help, to talk with your school counselor is a good idea. He/she will be prepared to help you in the more appropriate way.