To be totally honest I think it has very little with your BIL/SIL making the decision. I would imagine in most cases they would send the invite and allow the both of you to make the decision that best suits the two of you.
Also, by asking her not to attend I'm assuming it will be done in a pleasant way as to not iritate the sitch. I simply believe taking the control would be something that would empower you and may even lead to your W be curious, because she probably expects that you want her there.
You never did address whether you've actually discussed or know if your W really wants to go?
Anyways, again good luck.
BITS SIC
Me - 34 W - 33 M - 8 years T - 15 years D7, D5, D2 Bomb Nov 10/2010 "I'm not happy and INILWY" W Staying for the kids Mar 13/2011
