In "The Griffin Equivalency", a 2nd season episode of The Big Bang Theory, Howard, Sheldon, Raj and Leonard were dining at the Cheese Cake Factory (Penny is also present in the scene, of course). They were discussing about Raj being featured in the People's magazine.
Here she says:
Raj is gonna be in the people magazine. He didn't have to knock up one of the spare sisters.
So, who/what incident was Penny talking about there?