CW, I personally think that the OW should be introduced slowly, for the kids to go for a sleepover to her house when they have never met her seems crazy. Do I understand it right? Is he living with her now, or does he still have his own place?
He should start introducing her slowly..couple of hours at the time...dinner or outing. They need time to get to know her and get comfortable before he brings them to her house for a sleepover.
Could you have a talk with him and explain that to him? This is not about you (although I understand how painful this whole scenario is for you) this is whats good for the kids. IMHO
M53 H54 D17 M33Y T38Y Bomb OW 09/09 OUT 10/09 BACK 11/09 OUT 01/10 WANTS TO R 04/10 BACK with OW 05/10 Wants to Reconcile 05/11 I said NO