AIBU that we should pay for DM's meal?

[Peanuts2000]

My DM has a big birthday soon, we are supposed to be going out for a family meal. I think the children should pay for this which will include extended family, around 20 people altogether. One sibling thinks we shouldn't, only for DM, the others should pay their own. Feel embarrassed as DM wouldn't be expected to pay towards a meal if she was invited out by her extended family. Also know this sibling has the money, also DM has helped this sibling out in many ways including childcare for years. Other siblings are useless and don't want to get involved. Getting sick of the lot of them and feel like telling them to get stuffed!

Nope. I would pay for the birthday person and of course my own brood. But pay for 20 people? Not a chance.
It would quickly become a nightmare unless you put a budget of x per person including drink. As you know otherwise people will order the lobster, expensive steak, caviar and the expensive champagne. Even at 50 a head which really is about average, your are talking £250 each just for the extended family.

Although would like to go to the place that for £400 will feed 20 people a three course meal, booze, soft drink and coffee for £20 a head.. Actually scrap that.

For me it depends on the invite and the way it is worded.
For DC 18th birthday we had a big meal for extended family, we invited them all, formal invites etc, planned the menu (with choices) and paid the lot. This is the norm in our family.

However for the meal before DC went off to uni, I just invited grandparents my brother and her cousin. I made it clear that this was a family meal out and we were splitting the bill, which to be fair we would normally do when we go out, unless someone is feeling flush and wants to treat everyone.

Thanks for your comments, has been very interesting reading them. Seems it is split 50/50! I worked out we would have to pay an extra 60-70 pounds each for extended family. Update now is extra cost is going to be covered by DF, so the sibling is not paying any extra but then neither am I. Still not sure how I feel but at least the extended family won't be asked for money. Will see what happens!