Recently through a TBS rerun, I watched the episode with Sheldons sister. For me, this storyline was a real missed opportunity. Who better to provide more intel on the Sheldon Cooper background than his immediate and seemingly polar opposite family members. The boys go gaga trying to impress his sister, who makes friends with Penny and makes the boys sweat out each attempt to take her out. Since that episode, I do believe we havent seen hide nor hair of any other Cooper siblings. This is really unexplored territory, and something I think the writers will circle back to as we continue to learn more about Sheldon.