I've been on both sides.
My ex left for OW, wanted his own life, played games, late for pick up, abusive to me, wouldn't accept the routine of his contact, ‘no one was going to tell him when he could and couldn't see his own children, etc etc, but I stuck to it, maintained the positive, stood my ground and my DC’s have maintained a decent relationship with their DF.
My ex DiL - from the early days of marriage, didn't fully include us, didn't want to be part of our family, invited us to some events but made it clear we weren't really welcome. Some really poorly mannered behaviour which we tried to accept as ‘all families are different’ and not necessarily wrong.
Once divorced with a small DC, she destroyed the father/son relationship, frustrated the court agreed contact, repeatedly, blocked/unblocked family members, ignored celebrations. Eventually the court agreed that given the time that there had been no contact it was more damaging for the relationship to be re-established.
Even when we sent gifts to our grandson, we had no response, not even a thank you. Eventually this has been message to ask us not to send anything else as our grandson only knows us as a ‘name on a card’ and this Christmas had a near breakdown at the mention of our names…
There has been a sense of inevitability, right from those early days, that she wouldn't support our grandson to have contact with his DF or extended family.
Very sad that any person grows up with half of a family missing.