Hi, I was physically and sexually abused by my step-father between the ages of 11 and 17 (when he moved away). My own father was also mostly out of the picture. My mother constantly made excuses for my step-father's behaviors. I wouldn't have dreamed of speaking out to anyone about the abuse at the time (this was back in the '70's).
Please follow your instincts and your intuition. If your mother will not protect you, do not allow her behavior to dissuade you from protecting yourself. Speak up whenever you need to and set boundaries. Please do not be afraid to say to your step-father
What you're doing is wrong. If you can speak with a therapist or counselor, do so.
Best to you. I truly hope the best for you