Step Son

I took note sometime back of my underwear being caked in crusty cum. My stepson was using them to Jack off to and into. When we are alone he does inappropriate acts like walking around nude semi erect or fully erect. He’s 17 and has a beautiful physique on him and his body is beautiful to view but that is about all. He stepped his actions up. He knows our master bathroom has no door and one day while I was showering I was startled when he entered the shower while I was in it. He offered to wash my back his hands all over me and him again fully erect. He is much bigger than I but so is everyone. I’m 4’11” and 97 lbs and frankly he had little trouble holding me there soaping me down his fingers entering me. I was so afraid of being r**ed that I let him have his way with his hands and certainly did not reciprocate. I let him think that sex was possible if we left the shower but when we did I ran as fast as I could to the main bathroom where I was able to lock the door. He begged me to come out and promised to leave me alone. That was five months ago and he hasn’t done anything inappropriate. I never told his father but did tell my sisters who think I should. He’s the apple of his father’s eye. Not sure what to do but I will admit to being scared of being alone and worry what it would do to our marriage.