you guys need to listen to Love Line. Obviously you're only attracted to these girls because they're in relationships. Maybe you had a chaotic family life, or you were molested as a child or something...
(that was a Love Line joke, for those that don't listen...)
Anyway, yeah, don't tell her. That would be stupid. She's not going to break up with that guy to be with you. It'll only fuck up the relationship you guys have.