[M]y gfs mom

I’ve been dating my gf for the past year now and I’ve been eyeing her mom since we first met. She is insanely fit with a fat ass and massive tits. I have been really wanting to show my interest in her and possibly have something happen between us but I have no idea if she would even consider doing anything with me since I’m with her daughter. Please give me realistic suggestions on how to possible make this happen or if I should just leave it as a fantasy. Also if anyone has done this I would love to hear about it!