I don't know... This seems very dicey to me. My wife has beautiful, always-pedicured, ticklish feet. Her sister's feet are even more ticklish but not as nice and rarely pedicured. I've seen her tickled once or twice and can say that if my wife is a 7/10 on the tickle scale, she's a 10/10. That said, I would never tickle her. I think my wife would be furious and on top of that it'd be creepy to me and wrong to my brother-in-law. I certainly wouldn't want him touching my wife's feet. My foot fetish is such a part of my sexual life with my wife that I think if I touched her sister's feet in any way she'd feel betrayed. Tread lightly, OP.