Leonard's Sister

This woman is only daughter of Alfred and Beverly Hofstadter, as well as the sister of Leonard Hofstadter and Michael Hofstadter. She has not yet appeared on the show, but has been mentioned a couple of times. It is known that she is a cutting-edge medical researcher working with s to cure.

She is older than Leonard, as Leonard mentions in "The Cohabitation Formulation" that she is 38 years old and married, after Raj is mad at him for spending the night with his sister Priya and says in retaliation that he shall have his way with Leonard's sister.

In "The Celebration Experimentation", Beverly mentions that Sheldon and both Leonard's brother and sister were invited to the celebration of her 60th birthday. Leonard didn't even know about it.

Family

 * Father: Alfred Hofstadter

Trivia

 * She is the only member of Leonard's family of whom her relationship with Leonard is unknown.
 * Having mentioned she is married in 2011 ("The Cohabitation Formulation"), Leonard refers to his sister as Dr. Hofstadter in 2012 ("The 43 Peculiarity"). It is quite usual for academics to keep their own surnames upon marriage though some wives still use their maiden names for professional reasons, especially if they were established before they were married.