If your fiance's sister is already married and has two kids, do you have to have her in the wedding party?
Contrary to popular belief, being single isn't a prerequisite for being in a wedding party. The people who stand up for you should be those you feel closest to, no matter what their relationship or parenthood status is. It sounds as though you just really don't want to have your sister-in-law in your wedding party if you can help it. Maybe you two don't get along -- or maybe you just don't know her that well but feel obligated. Generally it's not mandatory to include your fiance's opposite-sex siblings as your attendants, but it is expected in some families. If that's the case, you'll have to decide if it's worth potentially upsetting your in-laws or if you should include her just to keep the peace. Also keep in mind that she doesn't have to stand up on your side just because she's female; she could also serve as a groomswoman on your fiance's side.
See More: Choosing Wedding Attendants , Your Bridal Party , Bridesmaids