If this is your BIL from your side of the family then there should be no reason for your wife to go. I would change the plans. She can't insert herself whereever she wants to. Make other plans with BIL and leave her wondering.
I am a newbie on this thread but your wife sounds like a cake eater...I have lots of experience with this. She wants to play married when she wants and then single when she wants. Can't do that!
Me: 46 FWS: 36 Married and Divorced 4/07, Pregnant 7/07,False R 7/07 Baby Girl born 3/08 Kicked him out because OW: 7/08 5/10 He realized what he had and lost. Moved home! REMARRIED 3/14/11!!