Fiancées Sister

I am engaged and my fiancée has a younger sister a couple years younger than her and I’ve fancied her since i met her. Maybe it’s because I’ll never have her, makes her more desirable but she has a gorgeous face, huge tits and a fat ass. I fantasise about her all the time and wank over her pictures regularly and check her out when I see her. I don’t know if I just forget about it all as I could never approach her as it would be far too risky and I don’t even know if she’d feel the same but I’d have a hard time saying no given the opportunity.