I know you're feeling some sort of “dark desires” as you have seen her naked. Ah, there are two threads here, one of them being regarding cheating, which is very rude and unfair to the person on your side. On the other hand, if you really don't want to be without your girlfriend, and ultimately want to have 'illustrious' things with her mother, try to be very discreet, because you might end up having what you want right there at the bottom, and the only thing that will be left will be a desire that has already passed.