I am only a casual fan of the show but I never heard about a possible brother of George. Or that he committed suicide, because that would go against what the show was about. There are no “Very Special Episodes” of Seinfeld. Not even close.
I always figured George to be an only child of very neurotic and mentally ill parents, and the pain of him having to live at home with them. I get that because I might be in the same situation in a couple of months in my life.