Perhaps your reading to much into the relationship between your fiance and his sister. The closeness they share is a sibling bond. When they are together, they may very well revert back to childhood, and the footsy thing under the table is something that they did as kids, kicking each other playfully under the table. His sister is married...does her husband seem uncomfortable with their relationship? Some families are closer than others, and every family is different. If you are uncomfortable with what's going on between the two of them, perhaps bring it up in a way where it won't upset your fiance. Such as jokingly bring up an observation like, "you and your sister seem like you are reverting back to kids when the two of you are together...I even noticed you guys doing the footsy thing under the table....hahahaha...what's that all about?" Then he may be able to shed some light on his actions. In any event, hope everything works out!!

I have to agree with you. My sister and I poked each other during prayers before meals as children. We also frequently linked our arms together. We just have a close sibling bond (we're 18 months apart). Even as you get older, you and your siblings still maintain that bond. The fiance and his sister probably don't realize that their behavior could come off as inappropriate to outsiders (read: LW). I wonder if the letter writer has a close bond with a sibling, but probably not since she isn't sure about her fiance's bond with his sister. Since it concerns her, however, she should ask him.