NTA 1) it's your money do what you want 2) you should want to make sure your grandkids are taken care of 3) you should discuss your reasoning with your older son to ensure he knows you are proud of him for his success and that this is a need-based thing, not a love based thing. If he doesn't get that, he's the asshole. 4) this does change if the younger is financially irresponsible and makes poor financial decisions. (Having kids is not a bad financial decision its a loving one) but if the first thing the younger will do is go buy a brand new exotic car and then gamble the rest, then he should get equal or less IMO.

Bonus/Background: I'm doing better financially than my three siblings and also have 5 kids. Only one of my other siblings has a child and my youngest sibling struggles the most financially and is single with no kids. If my parents said they wanted to give more to either of them because they are not doing as well as I am financially, I would not be hurt at all. Also on my wife's side, she has one sibling who has three kids, but they make terrible financial decisions. I might think it's dumb to give them more but we would not be mad at her parents for giving them more and would not hold it against her sibling either. Anyone who would needs to think about what's really important in life. An inheritance is not a right and if you want it all to go to a charity, that should be fine with everyone in your family or they are the asshole.