Well my side of the story is that I invited them to many events after the seperation, they did come to visit. They have family very close by where we live, so it is no skin off their nose to visit the children at the same time.
Now before seperation, they visited on Christmas Day the close by relatives and were long term planned to visit us after to eat their Christmas Day meal with us and my extended family. They were late so the ex rang them, as we were about to dish up, they had decided to not visit after all and were on the motorway home.
That is the sort of people we are dealing with here. They would never admit they would have left us with a burnt and cold Christmas Dinner and no phone call, or that they let us down. Their story at the time was that they were tired and just wanted their own bed and that the ex and I understood how tired they were. 
So these people don't want to tag the children on to a visit they make to other relatives near by, and goodness knows what their story is to people, I very much dout it casts them in a bad light.
The exmil has three other children who divorced before my ex and I divorced, and I heard the way she spoke about the grandchildren's other parent who was not her child infront of the chidren and I heard her gloat about how she devalued the money she spent on their gifts to "let them know how little she thought of them". When the gifts to my children were financially downgraded I knew exactly what she was telling them and me. They were very wealthy people, with several properties and good pensions. They sent a flannel for Christmas one year, the children were used to very extravagant gifts in the psat, they knew without me saying anything what that gift was all about. You can just imagine it the package came, they had to have it under the tree for weeks and then they open it up, all nicely packaged to get a flannel. It was binned.
Another occasion when a card was ripped up was recently one child's birthday was ignored no card etc, the next child's birthday was acknowledged, a card was sent, no name in the card in the address section, just the names of the grandparents, no love from or anything like that just their names. The child who got the card ripped it up in front of the other child who's birthday had been ignored and binned it, no more was discussed about the card. I thought that was appropriate behaviour and lovely sibling solidarity.