Welcome to PC.
Wish I had advice about your brother, but I've never had one and have limited perspective because of it. I do know that one of my closest childhood friends, had a falling out with her brother and they just stay away from one another. And how he has been towards her certainly seems at a more severe side of the spectrum. Because of that limited point of view, I say, sometimes personalities just don't mix very well.
I understand about the dizzy/near fainting feelings. I never got a full explanation from my doctors aside from attributing to an illness that I do have, but I don't fully feel it is, and thank you for mentioning sinuses...that makes a lot of sense. I wouldn't wish the experience on others, but it certainly would be nice to just share it one time to understand.
Hopefully others have more personal experiences with older brothers and can offer advice/experience/support.
"Investigate your hidden assumptions."-Cornel West