I think the normal way is for each generation to support the next until they are able to support themselves. How was Mil supported when she was a child? Most likely by her parents, that's when she got her payback. The choices she made about paying for good schools were hers not her child's so it is unfair to expect to be repaid. Giving help to elderly parents in terms of time when they get older is different but demanding money for luxuries is completely wrong.
I would suggest that BIL tells his mother he now has to make some sacrifices for the sake of his children and one of those sacrifices is no longer financially supporting his mother.
These two comments nail it.
It's all about showing love and care within families (which may include a financial element), but never to be expected, legalistic or controlling. The simple rule is that if you chose for a person to be born, you are financially responsible for them until they are an adult. If you never had any say in their birth (which, for somebody significantly older than you, could obviously never have been the case), then you owe them absolutely nothing by obligation.
My great-aunt married a man who was forced to give his entire income to his mother. The extended family also travelled each Friday to stay for the weekend and his wage was expected to pay for their food etc.
Absolutely crazy. How was he able to pay for his own house with no money - or was that all left to his wife (and what if she had had an equally abusive parent expecting all of her money)? Did the mother pay for all of the family food at the weekend from his wages that he'd handed over or was that considered a bonus? And, if the latter, how did they expect him to pay it when his money had all been taken already?
At least this would explain some of the threads on here where MILs despise their DILs for no apparent reason - I'm guessing a lot of them see her as a usurper of what they consider their own money (that they take from their sons). Most normal DPs and PILs see the wedding of their adult child as a joyous occasion where they're now making their own way in life, but some of these abusive parents likely see it as a 100% negative thing that's designed to hurt and 'steal' from them and kill their golden goose.
My DH is from a culture where he’s effectively responsible for his mother now that FIL has passed. So he’s bought and renovated her home etc. She has her own money, but he would tend to pay for everything unless she does something we don’t know about. This is quite normal for many people. There is also another brother who does the same. He lives in US. When she travels they pay for her flights and accompany her on the plane. If she comes in holiday with us, she wouldn’t need to bring money. It’s just how it is.
I may be wrong here, but I'm guessing that in this (and many similar cultures), it isn't really a case of support based on generations but on sex. The sons are only expected to become ersatz husbands (in terms of financial support) to their mums once their dads have died or can no longer work. Women don't ever have any financial responsibility, but of course, that means they also don't have any financial rights. Yes, she has her own money (many don't), but what use is that money if she's not really expected/approved to spend it? Is it considered pin money that she could use to buy herself some new clothes or cushions but it's not really within her remit to make major purchasing decisions (even if the money is plentiful)?
Effectively, many women are infantilised their whole lives and you end up with the situation that a 16yo boy who has started working has more financial power than a 100yo woman will ever have had in her whole life.
Apologies if I've misinterpreted this and I mean no offence to cultures where everybody is happy with this setup (if they indeed all are) and it all works for them, but it certainly isn't a way I'd choose.