Am surprised at the level of bad comment being directed towards the ex here. Yes, in an ideal world she would move on from the past, but we don't actually know how bad that past was do we? What we do know though is that her ex was enough of a shit in their marriage that she threw him out even though she had a newborn and a four year old. People are saying the DH has mended his ways, but what were those? Infidelity? Drugs? Gambling? Violence even? the OP has been very careful not to go into the detail of the past, I suspect because she knows that people would react differently if they knew just how much of an arsehole the DH was in his previous marriage.
And now the OP has got together with this man, and is very happy because he's not the man he used to be. But from the ex's point of view, the OP has drawn a line under the past because he's not like that with her, and rather than make her opinions known or say something she might regret, she instead chooses to say nothing.
In most situations the ex wouldn't be coming to family events. But I suspect that because the DH was single for six years before he met the OP, her coming to these events just became a patern, and her relationship with her ILs continued after the split. And given the amount of time between relationships, it would be unreasonable for the ILs to cut out the ex now that there was a new partner on the scene. If they'd chosen to do that after the split that would be one thing, but to continue the relationship with the ex and then cut it off when the DH met someone else would be wrong.
And it seems that all the ex is doing is not engaging with the OP. What's wrong with that? And it's not up to the DH to veto the ex being invited. His part in their split is more than likely responsible for the relationship which his parents have with her anyway.