""MIL rings OW for chats several times a week""
No she doesn't, she rings her current DIL and Mother of her GD, several times a week.
I don't agree with cheating, but if your marriage is over, it's best to split, even if there is a baby. It's best for the baby, rather than Daddy leaving when they're three (and them living with what's often described on here, when a couple don't even like each other anymore).
They should of waited before a pregnancy, ideally, but we don't know if that was for medical reasons.
The MIL is thinking ahead, all of her GC are half siblings and it will probably be MIL that will help with that sibling relationship, in the future.
It sounds as though she doing what she can for her GC and that is at odds with what the exSIL wants.
I've had a massive argument this week with my Son In Law because he expects me to get over involved when two of my adult DD are arguing, they're adults, they should be capable of sorting their own shit out.
You get to around 50 and you do your best for as little stress as possible, it makes you actually I'll as you get older.
It's very soon to be meeting her other children etc, but in the future when the all the children want to meet up, you may have to have a complete turn around.
The SIL could get another partner with children and/or distance herself from her children's side of the family, anyway.
You learn when you get older that things are competing different a couple of years later, it isn't worth having major fall outs and fights.
Tell people your opinion, but then draw a line under it.