No way. She's there to be sure you don't say anything that might put her in a "bad light". Did she just invite herself?
If so, I would just tell her it was a guys night out. Or have your BIL choose someplace where she wouldn't be caught dead in.
She wants to control the situation. And all I can see is BAD NEWS coming if this happens. If she insists on going, then tell your BIL you'll go out with him separately.
And besides she didn't want to talk about anything other than the kids right? I think with your BIL it would be much more than just the kids.
M-43 W-40 2D - 9 and 5
Emotion, yet peace. Ignorance, yet knowledge. Passion, yet serenity. Chaos, yet harmony. Death, yet a new life.