Again, one of many.
If you've never watched Seinfeld, just think about that friend that most people have who, in an attempt to console you after a breakup, will remind you that he didn't just cheat on you with one girl. He did it with six, so it's really his fault. This friend is constantly pitiable in his attempts to relate to a world that baffles, overtakes, and rejects him. This friend isn't so much a person as he is a "How Could This Go Wrong?" test subject in the grand experiment of life. George exists to filter potentially positive situations through himself until they become unrecognizable and awkwardly horrific. This would not change with something like the reminder of Pauline. And overcome with regret and guilt, the brother decides to end it all.
George's brother commits suicide, and it is essentially George's fault. Therefore, George's behavior, family dynamic, and almost all of the Costanzaic depressiveness of the show is given a shocking context.