XH left for OW when I was pregnant and had a toddler, they are engaged now and have regular (e.o.w) contact with both DC. It's been really hard to get to this point, but it's almost friendly between myself and XH now (slightly better than civil, try to be flexible and inclusive etc). I have no idea what his DP makes of me though as I have absolutely no contact with her. I am nice about her to the DC, as they have no reason to know anything unpleasant and they clearly like her which I think is a good thing.
As for MIL etc, my parents are good friends with MIL and her DH, so they go out for meals and stuff independently. I see them with the DC about once a month. I've just been invited with the DC to go out on a day trip for my DNephew's birthday, with SIL, BIL, MIL etc. I recognise that it's probably hard for XH's DP, but at the same time, I'm very grateful for all the love and support that his family showed me when we broke up in very nasty circs, and MIL especially has told me I'll always be family as I'm her DGC's mother.
I think it depends a lot on how the break up has been handled, and what the other issues are. I would never accept an invite to a family 'do' on that side if XH and his DP could go with the kids. I only ever accept if it's me taking them, or no-one. I guess that's my line really. I make sure they don't miss anything with cousins etc if their dad can't take them, but wouldn't ever want to go along to things together.
In XH's family there are lots of step situations, MIL is on 3rd marriage, has her own DC from two of them and is v close to her SDC from the second - so I guess it's all quite normal to them to just get on with it and be nice. Which I am very glad about really!