1) Was it mutual? I get the sense you wanted to say married but are now saying that because of the PA. Dig down deep and be honest with yourself here. If it was mutual so be it, but I agree w/LH...don't lie to your kids about whose decision it was.
2) Don't stand for your son calling your W a C.
3) Don't get into talks with W's sister discussing W's actions.
4) Your kids are going to be upset. They're also going to find out all about OM on their own and that'll likely make them mad at their mom. It's not your job to fix their relationship with her.
On #2, in Dink's defense, you cannot reason with a drunk. I think Dink actually handled his son really well.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018