The secret siblings of Seinfeld

...introducing a guest post from Mr. Chris Jaskot:

It would appear that all of the main characters in the show are only children, however that is not the case. Although never mentioned more than once, 3 of the main 4 characters of the show reveal a hidden sibling throughout the course of the show, always in passing and never to be seen or mentioned again...

Elaine's Sister: - "The Pick" - Season 4 - Episode 13 (nip slip)

Elaine: Oh, God - my nephew! (Picks up the phone and hits the button) Hi, Gail!.. Yu.. yu... Yes, Gail, I know how old he is.

Jerry's Sister: - "The Chinese Restaurant" - Season 2 - Episode 6

Jerry: Oh my god, Lorraine... that's Lorraine from my uncle's office. I'm in big, big trouble.
Elaine: The one you broke the plans with tonight?
Jerry: Yeah, she works in his office. Now she's gonna see him tomorrow and tell him she saw me here tonight. He's gonna tell his wife, his wife's gonna call my mother. Oh, this is bad, you don't know, the chain reaction of calls this is gonna set off. New York, Long Island, Florida, it's like the Bermuda Triangle. Unfortunately, nobody ever disappears. My uncle to my aunt, my aunt to my mother, my mother to my uncle...My uncle to my cousin, my cousin to my sister, my sister to me.

George's Brother: - "The Parking Space" - Season 3 - Episode 22

George: I can't park in a garage.
Elaine: Why?
George: I don't know, I just can't. Nobody in my family can pay for parking, it's a sickness. My father never paid for parking; my mother, my brother, nobody. We can't do it.