During its 12-season run, The Big Bang Theory mentioned Penny's sister on several occasions -- so where was she during Leonard and Penny's wedding

The Big Bang Theory was all about the dynamic between the core group of characters: Sheldon, Leonard, Raj, Howard and their girlfriends/wives Amy, Penny and Bernadette. 

Since the series ran for so long, it often widened its cast to introduce members of the characters' extended families. 

Indeed, during the sitcom's 12-season run, we met Leonard and Raj's parents, Sheldon's mother, Howard's half-brother and Penny's entire family. Well, almost her entire family.

Throughout The Big Bang Theory's 12 seasons, Penny brought up her sister on multiple occasions. The character was first mentioned in the second episode of Season 1, "The Big Bran Hypothesis."