Despite Missy and Sheldon being twins they are very different both intellectually and socially. Courtney Henggeler has only appeared in one episode of The Big Bang Theory - "The Porkchop Indeterminacy", but her character will be appearing again in season 11.Missy Cooper (Courtney Henggeler): Missy is Sheldon's twin sister who is intellectually and socially very much unlike him. Confident, tall and attractive, she immediately catches the attention of Leonard, Howard, and Raj. Sheldon realizes that within Missy's eggs lies the potential for another "superior mutation" like him. He temporarily elects himself as a guardian to Missy in order to choose the best mate for her (she rejects Leonard and Howard in favor of Raj, but his selective mutism derails any chance he might have with her). Missy objects to Sheldon deciding who she can sleep with, refers to Sheldon as "Shelly", and despite their differences she loves him and is proud of his accomplishments. Sheldon first mentions an unnamed sister in the first episode and again in the 7th season as an unseen character, where she gives birth to a son, and it is also mentioned that she is married.