The Guitarist Amplification - S3-E7
Sheldon Cooper: Leonard, when that woman moved in three years ago, I told you not to talk to her, and now look, we're going to be late for the movies.
The Guitarist Amplification - S3-E7
Penny: Wow, Sheldon. I had no idea you made a board game.
Sheldon Cooper: Oh, Research Lab is more than just a board game. As it says in the box, "the physics is theoretical, but the fun is real."
Leonard Hofstadter: We must not be playing it right.
Answer: Because Raj's selective mutism seems to be caused by women that Raj sees (on some sort of conscious or subconscious level) as a potential sexual partner. An attribute that wouldn't apply to either his sister or mother.