It being “ethical” and it being “okay” are two seperate things. Ethics is mostly subjective. Most people would say it is unethical though. It isn’t illegal, you have the right to divide your assets as you wish, so that would make it “okay”.

Differentiating based on mental illness and your perceived value of a person based on their mental health struggles is plain wrong and not based in reality, unless said person is at a point of needing a caretaker to get by. Especially since a bunch of illnesses are hereditary. Stuff like ADHD can manifest in different ways. My brother is both hyperactive and has attention deficit. I only have the attention deficit part. Both of us lead normal lives. Our parents dividing assets based on the fact that he has it worse on that front, and giving more to me, would be plain wrong and nonsensical. Especially since I’m absolutely shit with money, and he isn’t.