Fucking my Stepmom as has phone sex with Dad

I shouldn't have listened in on my stepmothers conversation with my father. I knew right away that she was having an intimate conversation with him. They do it often. I listen in of course but this is the first time that she has caught me. I say that however now I'm not too sure. She didn't seem too surprised when she looked over and spotted me. In fact, it seemed as though she enjoyed it. It spurred her on. It shouldn't have but hearing her talk dirty to him on the phone as her focus was on me drove me wild. I couldn't help myself.