My encouragement for you would be to work on your relationship with his kids and find ways to be interested in them together - it's a package after all; if you love him you need to find ways to love his kids too.
And of course, it's okay to talk to him about how to manage his relationship with his ex-partner better, but be open to supporting him to do that, and even supporting them to have more of a relationship if that is what is best for the children.
And while it can feel like the solution needs to be that this just all goes away, unfortunately, that's just a destructive fantasy. Blended families and managing stepchildren can be complicated and at times frustrating. But they can also be deeply rewarding, and loving him means loving all of his life, even what came before you.
Q: My daughter in law had a terrible childhood and now has fed conflict between my son and myself, and while they're still letting us see the grandkids, they've largely stopped talking to us and won't tell us why. This has never happened between my son and me before, should I try to push them to talk to us?
A: That sounds really upsetting and its hard to know what to do, especially when it's not clear what the exact trigger for the current conflict is. It sounds like you suspect your daughter-in-law is tangled up in things in an unhelpful way though, and you may be right.
When we come from backgrounds and families where things are conflicted, or even downright abusive, it can leave some expecting the world of relationships they inhabit to behave in the same way.
Patience and calm are called for. What we don't know is what conversations they are having between them about the current situation, and until things are a bit clearer it may be counter-productive to push.
I think it is okay to keep gently opening the door and calmly stating the truth from your point of view. "We're here when you want to talk, and we want to understand what we did and fix things when you're ready."
It's also good that you still have access to your grandkids, although - a word of caution - it's really important not just to keep them out of it but if they ask what's going on - as kids are likely to do - suggest they talk to their parents about it.
Ultimately, time, and eventually talking will lead to a resolution. It's going to be really important to not express your anger at your daughter-in-law, at least at first, because this will also likely inflame things. Try to see her point of view and the grain of truth in her perspective, and take the time to hear what has upset her.
It can be hard to get used to less conflicted ways of doing things if that's not been your experience of families. Hopefully, over time you'll be able to teach her about that.