I need advice should I tell my mom?

For context I’m 20yo college student. My parents got divorce when I was 10 and my mom recently remarried around 5 years ago. The guy was nice and seemed to really care for my mom so I had no issue with their relationship then two years ago my mom had my younger brother. And I go to college out of state so I don’t see them as much anymore but I do call my mom weekly if not biweekly. Everything was fine until earlier today, when I got texts from my stepdad. He called me asking if we could talk and I said sure. He then asked if could stay between us and I wasn’t sure what he was gonna say but I agreed anyways thinking he was gonna talk about my mom. He told me that there have been 2 or 3 times where he has had dreams of me in an in appropriate manner and asked me not to tell my mom since he knows it’s wrong. He then told me that he sent a picture by accident and tried to delete it but if it ended up sending to me to delete it and not tell my mom. This caught me very off guard and I don’t know what to do. My mom seems happy and I don’t want her to raise my brother alone again but I also don’t feel comfortable with the situation since I care about my mom and this is very odd. What should I do?

Edit: update can be found in the thread it’s a link