Carl Carlson's sister is the unnamed sister of Carl Carlson.
History
Every Valentine's Day, Lenny dates Carl’s sister and Carl dates Lenny’s sister. They like how they can date someone with their best friend's face.
However, in "The Saga of Carl", when Carl went to Iceland to see his adoptive parents, no siblings were shown or mentioned. Therefore, Carl's sister was either adopted by someone else, lived with her biological parents or lives in Springfield.
She is seen with Lenny (along with Carl and Lenny's sister) on the yacht in "Gorillas on the Mast".
