Google says the guy's worth $500M. A mere 1% of that in a trust could ensure that none of his siblings would have to live in poverty ever again. In absolute numbers that's a lot, but in relative terms I've given away much larger percentages of my net worth to relatives, including ones more distant than siblings, I'd do it again, and I'd think less of anybody who wouldn't do the same.

You don’t see any other possibilities? Not, say, paying her rent (or, realistically, buying a house with his spare change and letting her live there) or chipping in for therapy? His net worth is estimated to be in the half billion range so he’d never even notice the cost.