Interesting, so what body parts did your sister tickle you on at Thanksgiving, if you would be so kind to tell us?
I know I'm a guy, but from a male standpoint, I can understand your fiancee's point. If another guy tickled my gf or wife, unless he was a family member like a brother, cousin or such, I dont think I'd be too thrilled either, and I'm not even the jealous type. We men are a bit sensitive to that kind of thing. I dont think too many women would react well to their boyfriend tickling another woman either. Just my viewpoint. As for family tickling, you mentioned how your sister tickled you at Thanksgiving, that is so common that I would have no problem with it. Even if I loved my significant other to pieces, one would have to be the really jealous or touchy type if they got upset by a family member doing that. I think it would be funny to watch. Only my opinion.
Mitch