I think the phrase “playing happy families” does people who say or think of things this way no favours.
The alternative to playing happy families is what, a warring family? Should parents be forced to become essentially estranged from their children because their child cheated?
It’s not “playing happy families” it’s just people getting along, keeping the peace & a happy vibe for your children.
And they are not playing happy families. Your ex is navigating a relationship when he has kids & adjusting / accommodating to include your kids in his life, which at least right now includes this Woman.
I mean this gently but it’s been 3 years & you’re with someone else. It may do you well to stop calling her “other woman” & call her by her name.
Whether this woman told you out of spite or to inform you, she did in fact tell you. It seems like you have focused your energy onto her being the messenger of this unwanted, but vital piece of information.
It’s lovely that his family have been so close to you & a support, that they love your kids. Try not to now be passive aggressive & short with them, for trying to have a relationship with their son.
It is unrealistic to expect they would have ostracised this woman for life simply because children are involved here.
Your futures were not with each other. Both of you are now free to explore the lives which were waiting for you on the other side of the marriage.
Make peace, look for the silver linings & let go of the anger.