Whatever is being purchased comes to £250. OP knows and chooses all of this (in fact in the OP does she even say she does the food shop with her sister?) so the £250 isn't the issue here, it's not a surprise to OP what has been purchased, or that she is expected to cover 1/3 of the bill as it is pre agreed that it covers 3 households for all of Christmas Day.
Post event, complaining that you're having to pay for the agreed portion of exactly what you were equally responsible for choosing and buying, because, for example, you didn't drink as much as you could have chosen too, seems off.
And if it's going to be at the level of wanting a more accurate split over the actual consumptions, then the sister should be entitled to nitpick over the cost of her utilities and cleaning and consumables...plus the value of her time and effort for the additional prepping and hosting.
Depending on where OP is location wise, she could always decline the invite from her sister and cook for just herself much cheaper than £83.33, or eat out. Here, a Christmas dinner, on Christmas Day, in a nice local restaurant, starts at £110, without drinks. Probably available for as little as £60 in some of the cheaper places but then potentially questionable quality.
I think if it was a £200 discrepancy to quibble, then it's worth the discussion. Over £20.80, when OP doesn't mention once that she's struggling for money, is it worth the upset it's likely to cause?