The boys all end up competing for Sheldon's sister, Missy's affection. Raj and Wolowitz even get into a physical fight.
Added:
A picture of Sheldon and his twin sister Missy (Courtney Henggeler) who definitely seems to have gotten the looks of the family.
Added:
Wolowitz tries to ask out Sheldon's sister Missy (Courtney Henggeler) when it becomes his turn and he gets shot down horribly. Poor Wolowitz.
Added:
Leonard and Wolowitz try desperately to hit on Sheldon's attractive twin sister Missy (Courtney Henggeler) without much success.
Added:

The Big Bang Theory Season 1 Episode 15 Quotes

Sheldon: You see, I'm a superior genetic mutation, an improvement on the existing mediocre stock.
Missy: And what do you mean, "mediocre stock?"
Sheldon: That would be you

Leonard: So, how do you two know each other?
Missy: He once spent nine months with my legs wrapped around his head.
Leonard: Excuse me?
Sheldon: She's my twin sister. She thinks she's funny, but frankly, I've never been able to see it.
Missy: That's because you have no measurable sense of humor, Shelly.
Sheldon: How exactly would one measure a sense of humor? A "humormometer?"