From what you say, your DH's daughter sounds unpleasant. Why can you not be gran/nan??? And why on EARTH are you (and your DH) paying a quarter of her childrens school fees?
And £12,000 a year FFS! We don't pay any bills for our (adult) children, they're grown ups FFS. They pay their own way, like grown ups do. I mean, we buy them gifts, and pay for meals out sometimes, but not their bloody bills!
And whilst they don't have children yet, if they do ever have them, I don't see why we would be paying for their schooling. You and your DH need to stop paying towards his daughter's childrens school fees!
One of my adult DC did Christmas day AND Boxing day at their partner's family's homes (parents, grandparents, auntie,) AND Easter Sunday and Monday!!! They never had time to fit us in. (We live 18 miles away, and all of the partner's family were within 1-2 miles ...So they weren't keen on coming out here as they found it as 'bit much' with having to fit in the partner's family.)
We just saw them The Thursday or Friday before Easter, or 22nd or 23rd December and swapped gifts then. We were always disappointed and hurt that they put the partner's family ahead of us, every year. For the 7 years they were together. We never said anything though. Maybe we should have, but I guess we just accepted it (that we were second choice.) Even if there is a bit of a distance, they could come every 3rd year for example.
They've split now, and they have a different partner, and this year they're coming to us for Christmas day! I'm so pleased! 
Hopeully you can get this sorted @Nagramama You need to be firm and say you want them at yours for Christmas day for a change. If they say 'no' (which they probably will!) then don't bother hosting them at all (on Boxing day/Easter Monday etc...) OR as other posters have said, go away on holiday, and to hell with them all.
And withdraw the bloody funding for your stepdaughter's childrens schooling. I mean it wouldn't be so bad if she was a lovely person, and was nice to you, but she sounds quite mean-spirited. As I say, not even allowing you to be called gran or nan.
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