Season 1, Episode 15 - The Pork Chop Indeterminacy
Aired May 5, 2008
Sheldon's attractive twin sister, Missy, is in town and the guys can't avoid hitting on her. As they all vie for Missy's affections, Leonard devises a plan to eliminate his rivals. Eventually, Sheldon decides he should determine who can date his sister. Meanwhile, Raj takes part in a drug trial to try overcome his selective mutism.