There is a lot of back story here, so this could be long, sorry.
My PIL are toxic. Pretty hideous stuff, and I have no contact with them. The dc haven't seen them for about 4 years, i think, apart from the odd brief chat on Skype (has maybe happened 3 times in total), following a huge row at their golden wedding celebration (when I say at, I mean around the time, not brawling at the actual party!)
So, earlier this year, PIL started making noises about MIL's 80th birthday next Easter. Apparently she has never had a big birthday bash (FIL has had several), and FIL thought it would be nice for her to have a party. So far so good.
Of course, it isn't that easy, and to cut a long story short, FIL wanted dh and his brothers to pay for the party. It is their duty apparently 
. Not just any party, mind you, but she wanted a long weekend in London, at a private club, with relatives/friends flown over from Europe (at dh's/BIL's expense ), put up at the club, formal dinner, plus sightseeing tours and experiences (and other meals obviously). Stupid amount of money that we don't have to spare. To top it all off, when dh and I mentioned that this didn't really suit our family (we have 5 dc between us, 2 learning disabled, the other 3 too young to enjoy this kind of weekend, so not suitable for any of them - they make up a third of the grandchildren), and suggested a couple of alterations to make it bearable (theatre instead of one activity, afternoon tea at the Ritz/other posh hotel instead of one of the formal meals), FIL threw his toys out of the pram, said it was his way or no way at all, and that it wasn't about our children (true, but who did he want to pay? ah yes, us.). Cue MIL phoning us up, crying lots about us spoiling it all, saying she would have to see 'our lot' another weekend (she has never met our youngest child, and is showing no inclination to want to do so), and still expecting dh to foot a third of the bill, despite only him being able to attend.
Anyway, another big-ish row followed, and the upshot was that the party was cancelled (or at least stopped being discussed openly).
This weekend we received an email from BIL (dh's older brother, not the closest of relationships, but cordial enough at the moment).
He is proposing the following:
At some point next summer (a date conveniently coinciding with a big birthday of his ), the family should:
have a Thursday/Friday in London - dinner, theatre for whoever wants to go (ostensibly to celebrate MIL's birthday 3 months earlier)
then everyone back to theirs (huge house) on the Friday, for a formal dinner for MIL's birthday (marquee in garden, for 20 max, apparently - this is odd, as with all brothers and grandchildren, numbers reach 18, so presumably just family - why the marquee?)
On Saturday a BBQ, and local circus for those who want to.
Then, andthis is the bit where I immediately thought 'cheeky sod' - Saturday night, a party for around 100, in the marquee, to celebrate BIL's birthday.
BIL wants the costs shared, as it is for MIL too.
AIBU to think that BIL has just tacked on the bits for MIL (considering family would be there anyway if he had a party on the Saturday), mentioned the circus etc to make it seem like a weekend of events, when in fact he wants everyone else to stump up for his birthday party? He is just as bad as PIL, isn't he? (well, to be fair, he hasn't actively banned young children, and has considered them in his plan, so not quite as bad). He has been upfront and said he can't afford a party by himself, so won;t have one if we don't agree to share costs, but I am still left with thinking 'why would we pay for this?'