I guess how I felt would depend on if my sister knows? I wouldn't keep it from her. I'm good friends with my husband's ex-wife's sister, similar? LOL However, I also like my husband ex-wife almost better than he does, try to include her in things more than he does. But his ex-wife and her boyfriend, his ex-wife's mother and sister and sister's boyfriend come every year to our house for Mother's Day (with their three grown sons and wives/girlfriends and grandchildren, etc.).