Wife's Sister Is Hot

My wife's sister has been flirting with me on an increasing level. I've been responding like any red-blooded man who thinks too much with his organ. She makes comments to me about, "If I ever get divorced, I'm coming after you." She saw my package (I'm gifted by nature) once when I changed into a swimsuit on the boat and she's made a few comments about it. She's very hot, and I'd really like to do her, but am worried that it would hurt our marriages. On the other hand, it would be so fun to have sex with her because we're more alike in the sexual department than either of us are with our spouses.

— Grover, 47