[AOEforme]

A suggestion you could make to your future SIL: hire babysitters for the evening.

We're doing this for our wedding, because my family has a good amount of young kids and we're having flower girls. We have two family friends who are in high school but not quite close enough to us to be invited to the wedding. They are excellent baby sitters so we're paying them $75 each (big money for a 15 year old) and for their hotel room.

We're letting our family know that we're providing sitting services for the duration of the reception, so they can bring the kids to the wedding (if they so choose) and drop them off in the room after dinner. This way the kids get to play with all their friends, swim in the pool (both girls are lifeguards), and everyone gets to enjoy the wedding. Also, we get an "adult only" feel without having to exclude family members.

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Also, I would honestly give her all the reasons you listed (politely) and then volunteer to help with other things, especially the cookie table. (I always feel bad asking people to help with mine, because it's asking someone to bake for 300 people).

In addition, she may actually have another girl (not to be rude) that she would rather have in your place, but bumped per your brother's request.

Finally, you can always tell if someone doesn't want to be in a wedding party. You wouldn't be able to do it with the excitement every bride wants in her bridesmaids and it would end up sucking for both of you. I know I might be mildly resentful of a sister-in-law who was less than thrilled to stand next to me on my big day.

Tell her you appreciate the gesture but just can't do it and give her a different task you would be more than willing to do. Everyone wins!