This could be long, sorry.
Basically this year it's DD1's husband's family's turn to host Christmas, we alternate between me doing it and SonIL's parents doing it, their turn this year. In previous years, it would be DD1 going to SonIL's with DGD, myself and DS, DD2 would go to her DP's that year and then she and DP would come to mine the year I hosted Christmas if that makes sense. DD2 and DP don't have any children, DD2 sadly found out earlier this year she can't have them due to a medical condition. DS is grown up but much younger than the DDs and single, so tends to spend Christmas with me and DD1, either at SonIL's parents or mine. Hope that makes sense so far!
DD2 and DP separated a few months ago very messily, exP became nasty once it was confirmed that DD2 couldn't have children and it ended badly. DD2 is suffering the effects of this, no official diognosis as I can't get her to the doctors but DD1, DS and I are worried about her, we're keeping an eye on it.
Since the separation, DD2 has been made legal guardian of a friend's daughter for various complicated reasons, she hasn't adopted her but this is the long-term aim, depending to a point on what happens in the next few months/year. I've been quite heavily involved with the little girl since DD2 is now effectively a single parent and she's a real sweetheart, fits in perfectly. DD2 was invited to SonIL's parents for Christmas as she obviously won't be going to exP's. DD1 asked her DP (SonIL) if her DD (friend's little girl, not official yet but going to be the easiest way to refer to her) could come too, he said that was fine.
Earlier in the month SonIL's father was taken ill with appendicitis and rushed to hospital, he's been back at home 5 days now and still recovering, it's a slow process due to his age but he's expected to make a full recovery. I offered to take over Christmas this year but SonIL's mother said no, she would be fine to do it at hers as long as we all helped out, which I was fine with as that's what happens anyway, if I end up doing more than usual at hers this year then that's fine, I don't mind at all.
So SonIL's mother has phoned today to say that she's thought about it, and actually she's happy to have the usual lot from my DD1's side of the family over and DD2, but not DD2's DD because she's 'not family.' I was absolutely furious, began explaining to her that actually she is, at which point she said she had to go and put the phone down on me 
So now wondering how on earth to play this one! Advice would be much appreciated.