It doesn't matter if your BIL knows how to "handle her" because YOU don't know how to handle her.
You have a very golden opportunity here to let her know she is not in charge, she will not include herself in plans that involve your family unless she is invited and it simply will not happen.
SHE is the one that has been adamant that you only discuss the children. Since this is an adults only dinner then discussing the children would not be a topic of discussion therefore it would cross the boundary SHE set. If she has something to say about that you can gently remind her SHE is not the only one with the ability to set boundaries and if you honor and respect her boundaries then it's high time she start offering you the same courtesy.
Your wife is crafty and sneaky and you are probably right, she does in fact want something. Just let her know she won't be getting it from you or your BIL.
If you need some courage to make this clear to her just remember how RUDE she was about her b-day, rubbing her party in your nose and not including you or all the times you were told to wait in the yard because her mother can't stand the sight of you. Recall all the babbling that you were foolish enough to listen to about her sugar daddy that was so interesting to her. Don't bring any of that up but don't forget it either. Do not reward her bad behavior.
This is not her reaching out to you - this is about HER and whatever motives she has up her sleeve. You are on the cusp of being able to start nipping all this in the bud, don't lose sight now.
