I'm a and I think my girlfriend is into my mom. They made out once. What should I do?

My girlfriend and I have been dating for about 2 years. We met in high school. I'm 18. One time we were drunk at my house and my stepmom joined us. She's good looking but she's like 35. Someone dared them to make out and they did. I'm scared I'm going to lose her. Should I move out or break up with her?