I agree that I think for the time being the future ex wife is still in a sense your family. I think that it's a bit inappropriate for him to be bringing the friend to a family thing so soon, especially with the dynamic of the situation.
I may be a bit biased here as I was the ex wife who's best friend hooked up with my ex husband. They were having an affair and then we got divorced. I was crushed. I'm not saying this is right or wrong, but it made me feel less like wanting to maintain a relationship with my ex in laws that they welcomed the new girl with open arms.
Even if the marriage is rocky the future ex wife will probably be crushed and you don't want to be in the middle of it, which you will be. I like Angela S's answer.
Good luck that is hard. I hope your brother is considering everyone's feelings here. I still at times mourn the betrayal I felt. :-)