Some families do Christmas this way because only one person has a house big enough to accommodate everyone and therefore always does Christmas. It wouldn't be fair for that person to have to suffer the cost year in year in whilst nobody else pays.
So the issue is not that they charged, but that they overcharged/increased the charge. If everyone agrees that one person will host and that you all split the cost, then that is fine.
It should be worked out fairly in advance, ie you all just pay an equal amount to share the family cost, or else it is worked out per adult and per child and then you pay according to how many of each in your family. It doesn't seem fair that 2 adults should pay the same as 2 adults/3 children , however if you all agree that you are just contributing equal amounts and are happy with that then it is fine.
However, if one person is buying it all, then that person should be able to provide a list/spreadsheet with all the costs on, so that everyone can clearly see what the money has gone on.
If she advised £50 in the beginning, she can't just suddenly double it, especially if you also then had to take your own drink.
YANBU to pay her just £50 and the other family members should do the same.
£50 doesn't seem a lot for everything though if she had actually paid this. I go to family and always take 3 bottles of prosecco (£18 on offer this year), tin of sweets, pigs in blankets, Christmas pudding, dessert, crisps etc, so easily £40-£50 for my contribution towards Christmas/Boxing Day for me and my DC. That is my choice to take those things otherwise I would not be spending a penny on Christmas.