Conversation so far:
What does the show Seinfield  tell us about Cosmo Kramer's family?
Cosmo Kramer does not have a well-developed network of family members shown in the sitcom. Kramer was estranged for a long period from his mom, Babs Kramer.
Is Cosmo Kramer's father shown or mentioned in the show Seinfield?
In The Chinese Woman, Cosmo Kramer mentions that he is the last male member of his family, implying that his father had died.

Question: Does he have any siblings?
Self-contained rewrite: Does Cosmo Kramer have any siblings?