Have you ever spoken about retirement planning with him? I would not be shocked if you end up finding out that he hasn’t been spending too much and steadily putting money into funds wisely … He may be better set to live within his expenses than your sibling!
Re-reading the OP after all this discussion. So far, the best advice in the thread seems to be to leave your children out of it. If you want to support your daughter’s in-laws, set up a trust for them now, while you’re still alive, so that money isn’t part of the estate. Then leave money to your children equally.
Or, consider that you’re really not responsible for your daughter’s in-laws. There also seems to be quite a difference in the two situations: the brother-in-law will be a dependent forever. The sister-in-law has a mental illness, but is also an adult with a child. Supporting the brother-in-law seems like something your daughter will want to do because she’s in one of the best positions to do so, and somebody must, and helping fund that seems great.
Mental illness, on the other hand, is a whole different kettle of fish. I’m not saying to throw the poor woman out in the cold, but supporting her is going to be pain, drama, and heartache, and a lot MORE expense than supporting the brother.
So I’d say leave the money equally to your three children, having taken out a separate amount to set aside for the care of the disabled brother-in-law. Instead of encouraging your children to donate their inheritance to their sister, encourage them to donate cash to local mental health resources and tell their sister they are doing so. That helps the sister-in-law and family harmony without mixing inheritance issues into it directly.
In their specific cases I do know their scores.
But you’re certainly right to question the implicit assumption that low income → low assets and high income → high assets. Like you, I know bunches of folks at either end of the W2 spectrum whose assets belie their W2s. Small W2 w relatively big assets is a very comfy spot to land. The converse is not.
In the abstract, yes, this is absolutely right. OTOH, the OP realizes that his love for his daughter makes him want to protect her from her in-laws being a financial anchor dragging her to depths she would struggle to handle on her own.
There’s a lot of wisdom in this thread, and I’m not sure there’s an absolute right answer, although some are definitely more right than others. As I read through the responses, however, it makes me wish I still had a brother with whom I could look forward to fighting over an inheritance. He passed several years ago at age 57, predeceasing our parents (dad now also gone). He had no kids of his own, but two step-kids that I understand will be left a token in our parents’ (now mom’s) trust.
So we’re gonna keep this up until the ONE THING the OP is certain of is that he wishes he never started this thread, huh? 
Ain’t that the Doper Way?
As a counterexample, my father appointed me as trustee. We were in California, where trusts are standard. The trust was great because it kept my evil stepmother from looting more of my father’s assets than she did. My brother and I had zero disputes about it. I think a trustee would have been expensive, especially if my stepmother made problems. But the estate was relatively small.
Yes, I wrote him and here is his reply:
“Yes, I agree this makes sense. I will do it!”
The point is that I know my son very well and expected nothing different. My wife is having cognitive problems and I did not want to involve her, which calling him likely would. I trust him totally to carry out my suggestion. I really rather suspect he would have done the same thing even with my input.
I did ask my wife, “Have you ever known any of them to do anything mean?” She could not and neither can I.
There is!
This is basically what happened after my father-in-law died. My husband disclaimed his portion of the inheritance, which had been set to distribute everything equally. The person who wanted the house got the house and the money (not much) to take care of the house and keep it in the family for the next generation. No one wanted to sell the house, and having a house with three owners is messy.
It would have been better, and probably cheaper, if the father had updated his will.
My parents gave me a copy of their will (binder and USB stick) and we had a discussion about it. My FIL never wanted to talk about money, including his will, and left a small mess behind because of it. Fortunately his children are on the same page.
Glad it worked out as you hoped, OP.
Just curious - you gonna ask the other son the same thing?
I stand by my advice. You were lucky, but there is nothing like money issues to cause family disputes. My wife thought she was on fairly good terms with her sister until this blew up…
Also glad you are happy with the outcome.
Again, personally, I would be more transparent to all my children about what my concerns are, and were I your eldest would not want to be placed in a position that my sister would feel beholden to me just because I followed through on dad’s wishes. I’d be asking my dad to gift it directly himself and have it be a legacy of generosity from our parents, not from me. For the selfish reason of healthy relationships between us as siblings.
To me the details would be what is the best way to do that gifting. Now annually at gift limits to my daughter expressing the wish that this be used to build a reserve to help with future in law ethical obligations? Possibly encouraging them to set up a trust for that purpose? Or direct as a trust? 100% that I’d be making clear my wish that my adult children always have each others’ backs for support without resentment as needed of any kind be it money, time, or emotional.
Not advice. You know you and yours. Just opinion.
Not eldest. Sorry. Your wealthiest.
I’m not saying you are wrong, just that in some cases it can work out. There are plenty of cases where it doesn’t. The risk is spending the estate in dividing the estate.
You have to use your own judgement.
Apparently banks and lawyers are rather reluctant to take on trustee duties nowadays, and costs have to be considered, obviously. Just saying from experience that matters of money seem to bring out the worst in some people…
No. When he sees what his bro is doing, he might do likewise. But while he is comfortable, the money would mean something to him, while it wouldn’t to the wealthy one.
So, maybe not all quite so much on the same page, after all?
As I said, money issues are a major source of family disunity.
You raised some good kids, @Hari_Seldon. That’s fantastic and you must be so proud.
I hope I can also raise my kids to be kind and trustworthy!
We are, we are. When friends ask us what method we use to raise them, our answer is “benign neglect”. But they were such a pleasure to raise! And we never had to bug them over homework. Or pay any mind to it at all. Our daughter was #2 in her HS class. The sons were each #1. We consider ourselves so fortunate.