It’s very hard to be the kid in this kind of situation. You have very little say in what happens to your family and I can tell that it’s breaking your heart.
As you pointed out, issues around money have been a long-standing problem for your parents. However, it sounds to me like the fight is about more than money. It’s also about trust and control and relationships with extended family members. It appears that your Dad’s gift to your uncle brought those issues to the surface. If your parents are to have a happy or at least reasonable marriage, they need to work out all those issues.
You can’t help them with that. All you can do is let them know that you love them both and that you want the family to stay together. Ask them to keep you and your brother out of the middle. You can’t be helpful and it’s unfair to ask you to take sides. Encourage them to see a marriage counselor to find out if they can resolve their problems.
Whatever happens, please remember that their unhappiness is about each other, not with you. You can still love and be connected to both of them. A divorce doesn’t have to change that.
I wish you well.