D.H.
Unfortunately it is usually the brides choice for the flowergirl...it may be that the bride does know the cousin and she lives close to your son and that is why she was chosen. The best thing you can do is explain to your daughter that her brother loves her, but it wasn't his choice. If she was in the wedding, then you'd have to deal with his mother also...I'm sure that wouldn't be fun for you or your husband. Be thankful that you are invited to the wedding...go with your best game faces on and remember that it is his and his brides day. Be the bigger person from your son's side of the family...trust me people will see that you are gracious and not there to cause trouble and will appreciate that. No matter what his mother may do, grin and bear it. I've been to several weddings with step family, including my own and when the "other" family stood back and remained annonymous, things went well...when they tried to be "a part of the event" things didn't go so well and made everyone uncomfortable. Hope this helps. Good luck and God Bless.