I used to do hosting of a massive family Xmas, with financial chipping in from BIL & SIL, MIL, other SIL & DN.
The reason being that XH & I invariably hosted because we had the biggest house. We also had small dc, so a PITA to travel. We also had - in me - definitely the best cook in the family.
Yes, people also brought food & drink, but non essential stuff (extra booze, mince pies, snacks for buffet table). I wouldn't have trusted any of that flakey lot to actually remember anything that was important - & who wants to be suddenly sourcing all the veg on Xmas Day because their ditziest IL has forgotten they agreed to bring it?!
Well off SIL & BIL made a reasonably substantial financial contribution, which they were very happy to do (because they weren't having to host which they would both have hated) & which would be discreetly tucked into their Xmas card. Less well off SIL probably slipped one of us a token tenner. MIL would do similar & be firmly handed it back because she was the family matriarch & had done her share for the preceding 30 years...
Everyone was financially solvent, so it being a discreet voluntary bung was fine - XH & I could have afforded not to have anyone contribute. Equally, none of the relatives would have felt that was quite fair - it was always me & XH who hosted, for a decade!
If we'd all been skint I'm pretty sure everyone would still have preferred to cut things back a bit & put contributions on a more formal footing, rather than forego a big lovely Xmas dinner.
Anyway OP - I'm with everyone else pretty much. Send her the originally agreed 50 quid & draw a line. Do something else next year!