I've been a step mum for 26 years, so I've seen DHs three children grow up, get married and have children themselves. They are good kids and always have been.
Unfortunately their mother is a powerful matriarch and, without wishing to throw the word around needlessly, a narcissist. They are all inextricably bound to her by duty and loyalty, whether they like it or not, and as children they rarely even exchanged "hello" or "goodbye" with me as presumably I was the enemy. One said to me "You're not my family" when he was 6, which was true enough. I was very, very careful not to overstep and ignite their mum's fury, which meant stepping right back and ensuring that the focus was on their relationship with their father (which she has also tried to sabotage over the years, just because she can). I was nice to them, enjoyed days out with them, read to them, played with them, all the usual but I'm not their mother and they didn't need another one.
As adults now they are always civil and friendly towards me, might permit a perfunctory hug, I do occasionally get a happy birthday and even a mother's day text from them. Their children call me Granny although I don't see them often. I'm included on the various family chats and generally my messages are acknowledged, as a courtesy.
But ultimately the relationship is cool and dutiful, not warm. I strongly suspect that they wouldn't piss on me if I was on fire, and if DH dies first I'll never see any of them again.