hi OP, this may not be your experience at all so please disregard if there's nothing in it for you.
my ILs are lovely and would like to do the same as you. They are wealthy and have offered us lots of money. At the beginning, I felt really weird about it but my husband said that has always been the dynamic, it's how they show love and his mum loves feeling helpful and useful.
it really isn't healthy though, I ended up feeling like we couldn't say no to anything they offered or wanted to do because of how good they've been to us. Even though no one suggests it, the feeling is that we owe them.
they want us to go on hol with them every year, they pay for it and we have to come up with a 'good enough' I.e if we can't do it in June, when can we? If the place doesn't suit toddlers, what will?
they want to pay for kids activities, health insurance, education - of course that's lovely but it feels controlling and like they know what's best, like they want that for our kids and will make it happen.
we suggest they spend the money on themselves, enjoy themselves - we have never asked for money - but they insist they want to spend it on us and 'see us happy' but our family being their only enjoyment is smothering.
there's also the infantilisation of my husband and, in a broader sense, us as a family. Yes it's true that we can't pay for the luxuries all the time but we're grown ups who can do without like everyone else.
the 'I can't bear to see you struggle' is actually about my mil not being able to tolerate that feeling not about us.
as a DIL I would also be annoyed if my ILs assumed they knew how money should be spent in my family. If they are not asking for money, your assumption that they need it is insulting. The lifestyle that you think they want may not be what they want.
the reason I say this is because of your sister saying the word 'intrusive' and that landing with you.
it sounds like you are lovely and love your son and his family, you already do so much - you're of value without the money.
can you put the money aside anyway?
can you tell your son that you trust and know that he is able to take care of his family without your financial help and that you won't assume to know what he needs but he can always ask.
i know i sound ungrateful, i am grateful and i love my ILs but i wish they weren't so transactional. I tell them we love them not what they do for us - sometimes though it really annoys me how intrusive they are and how much they think they can control and suggest because they have given us so
much