I slept with the son of my boyfriend's obsessive ex. Smart move?

My boyfriend's ex has been coming after my boyfriend again recently after a divorce and won't leave him alone..(my bf even accused her of stalking him). She's your typical fat, middle aged hoe bag trying to pathetically recapture her youth which probably explains why him having me, a hot 23 year old as a girlfriend didn't do much to deter her. So I decided to shut her up once and for all by sleeping with her 18 year old son..(who is actually quite handsome). Now she's really pissed off as I have stolen both of the men in her life. How can I keep my boyfriend from finding this out as I'm sure she won't be able to keep her mouth shut for long?