First post, not sure if this is the right place but need advice as to whether I'm the one BU here.
I'm the DD2 of the MIL's selfish Christmas arrangements thread, Mumsnet has been recommended to me after my own mum felt so supported on here over that :) If you haven't seen that thread, a brief summary of events is that my Mum, brother, my foster DD (slightly more complicated than that but for lack of a better word) and I were due to go to my sister's ILs for Christmas this year, but two days ago my sister's MIL decided she was uninviting my DD as she 'wasn't family'- slightly more complicated than that but I'm trying to keep it short! All hell broke loose and the result is that me and DD, my brother, my sister and her DH and family (the MIL's son) will all be coming to my mum's for Christmas instead, my BIL is refusing to go to his mum's on principle.
We were all invited to drinks at MIL's tonight, but my mum, brother, DD and me didn't go because we had already booked to go out for DD's birthday. BIL is round there now with my sister and their DCs, apparently the atmosphere is not as bad as they'd feared but us not being there for Christmas tomorrow is the elephant in the room. There are other family members of the ILs, friends etc also round there, it's very much a low-key party.
DD and I are now home, my mum and brother have set off for mass so won't be going to the drinks- timing stated as 7.30-10.30 so we'd all missed the beginning of it out for DD's birthday. I've come home to a voicemail from MIL saying if we get back before 10 it would be lovely to see us at hers. This is after rather unpleasant things were said yesterday about both me and DD.
My gut feeling is no, don't go, but then I'm worried she's trying to make amends and I'm going to be making the rift between my family, BIL and co. and MIL worse. AIBU to completely ignore her and pretend we're still out? Or is that rude and resorting to her level? 
Possibly a bit more of a WWYD, sorry, but advice would be much appreciated.