Conversation so far:
Does Cosmo Kramer have any parents? Who are the parents?
Kramer was estranged for a long period from his mom, Babs Kramer. His dad never makes an appearance.
What about Cosmo Kramer's dad?
Kramer mentions that he is the last male member of his family, implying that his father had died.
Did Cosmo Kramer have any siblings?
Was there any other family member mentioned?
He also mentions in that he has or had a deaf cousin, from whom he learned fluent American Sign Language.

Question: How was relations between the two?
Self-contained rewrite: How was relations between Cosmo Kramer and the deaf cousin?