Before anyone considers "chipping in" have his finances been thoroughly and properly reviewed to check:
1 he's receiving everything he's entitled to - is he getting pension credits or other income that he should be?
2 his outgoings aren't unnecessarily excessive - is he on the best/cheapest options for:
gas/electric - switch him if necessary many pensioners are paying more than absolutely necessary for this
water (if privatised where you are)
TV/phone/WiFi (if he has - if he doesn't have WiFi you could well find that this is actually meaning his phone costs are higher than necessary, he doesn't need to use it though I am sure the siblings/grandchildren would find it useful) - weirdly these days it's often cheaper to get packages that include more than just have a phone contract
Mobile phone - if he has one is he on the best contract for his usage?
Council tax - has he made sure he's getting the single person discount following wife's passing? People often forget such things
Insurances - if he has contents and buildings ins (I'm assuming yes as he owns) is it the most appropriate and cost effective for him? People very often don't change this from when they bought the place!
Groceries - again is he doing grocery shopping cost effectively? Doesn't necessarily mean he has to cut back or change to brands he doesn't like (though it can be good if possible to get him to try cheaper options and find they're actually ok) but is he shopping in the more expensive supermarkets? If so perhaps persuade him to use cheaper ones, to shop around? How about loyalty cards and vouchers is he making sure he has and uses things like this?
Other shopping - as per groceries is he shopping in higher end/more expensive shops when there's really no need? Is he being savvy and taking advantage of loyalty cards and special offers?
I mention all of the above as we had similar issues (though they weren't living in 6 bed mansions which I have to be honest and say is bloody ridiculous for a single man! Aside from the money it must be exhausting cleaning and maintaining it!) with my grandparents.
They were pretty savvy with the shopping side of things being they were working class and never really had loads of money, but they were inexperienced and unaware of things like the benefits they were entitled to and not claiming and also didn't understand things like energy switching.
One gran once we had persuaded her to LET us review her finances and overhaul it for her was over £2000 a month better off as a result! Mainly due to reducing energy and insurance bills rather than increasing income.
As I said utterly ridiculous and not sustainable for a single man in pensionable years to stay in a 6 bed property purely for the sake of family meals a couple of times a week.
I'm from a slightly larger original family (3 kids) and a HUGE wider family (mums 1 of 6 dad 1 of 5) and my grandparents were in much smaller properties and yet I very fondly remember many Christmas dinners and other occasions at theirs where everyone attended (so all their children, plus spouses/partners, plus the grandchildren) and everyone was fitted in and had a rare old time!
they also all bring their kids, so it's a total of 14 people.
Mums side at the maximum it was 25!! Grandparents, great gran, mum & siblings, partners and spouses and grandchildren and that was in a 2.5 bed tenement flat in glasgow!
Even if he downsized to "only" a 3/4 bed he'd still have PLENTY of room for these occasions.
It's simply a case I think of finding somewhere that may have smaller and fewer bedrooms but the downstairs/living area is open plan/large enough to accommodate.
Have they or you even looked at any other properties in the area?
My parents are in a small 2 bed and the dining table still seats 8 - 10 comfortably. Perhaps they need introducing to the concept of a "kids table" as so many of us accept as normal?
I think they had a look and the houses that were more affordable tended not to have separate dining rooms, and the living rooms wouldn't be big enough for all of them I suspect they didn't look that bloody hard!! Bed they only looked within the immediate vicinity too.
My mum has a brother that is an arse about only living in one particular expensive part of glasgow and absolutely wont consider even a mile outside that, stupid and unnecessary snobbery that means he is in a house that's really too expensive for him and paying higher council tax than strictly necessary too.
Also agree FIL needs to be put in front of a recommended, clever financial advisor before any decisions are made just by siblings. Options will open right up when this is done totally agree!!
I do also agree them all getting together twice a week every week is utterly bonkers! This is not normal!
Most families get together like this only for special occasions - milestone birthdays and anniversaries and Christmas. Which within a large family ends up happening fairly frequently anyway, certainly most months which is more than enough for such meet-ups.
If it's a case of wanting to keep a subtle eye on "dad" to ensure he's coping and not getting too lonely etc then even each sibling taking it in turn each week to go with their WHOLE families inc partner/spouse, as there's 4 of them that still means he gets a weekly visit from someone.
In terms of FIL's wishes, I'm not sure that they've really asked him. I think he's said "the bills are getting a bit much" and the siblings have swung into action so it's entirely possible HE wants to downsize, the house may be starting to prove too much for him cleaning/maintenance wise?
And that's while he's relatively young (as a pensioner) and still in fairly good health.
My parents are now in their 70's and both in pretty poor health and very much regretting not moving to a bungalow years ago as they now really struggle to manage the stairs and bathroom even though they've made a number of adaptations.
He could well be thinking he's going to be facing the same problems.
It's even MORE ridiculous - and unfair - for him to stay in a house he can't cope in because your dh and his siblings are hanging onto sentimentality!
They need to wise up! Seriously!
I'm beginning to think you need a meeting with the other partners/spouses and see what they think too.