OP, you really have my sympathy. I used to be a solicitor dealing with exactly this kind of thing, and now help families find their own answers. Inheritance issues can cause so much upset. Of course one should never ‘bank’ on receiving money, and it is the parents’ money, but at the same time it’s normal for parents to leave their money to their children, and it’s normal for children to hope that an inheritance will help towards the mortgage/pension/school fees/etc.
When a parent is controlling, you can’t persuade them to be otherwise; and for an elderly/unwell controlling person, money sometimes becomes the only way they can continue to control their children after they die.
You might know why your father and brother don’t see eye to eye - is there anything your brother would want to talk to your father about while he is alive? Otherwise, I agree with other posters that it’s no good trying to argue your brother’s case, but you could have a different conversation which helps you too. You could just ask your father what he would most like his chosen heirs to do with his money in future. That might lead to a conversation where you ask his advice about grandchildren inheriting a lot of money while still young; or where you (gently) ask him to think about where you/your brother are coming from. You can’t make him change, or put your views strongly, but you can ask non-judgmental questions about what he wants or thinks, and that might make him think further. Ultimately, it is his choice and his money, but you might all feel better for having talked about what he wants to achieve.