The Vegas Renormalization - S2-E21
Sheldon: While my compatriots are in Las Vegas, I will be enjoying a blissful evening in my personal Fortress of Solitude.
Penny: That's Superman's big ice thingy, right?
Sheldon: You know, I'm in such a good mood, I'm actually finding your tenuous grasp of the English language folksy and charming today.
The Vegas Renormalization - S2-E21
Sheldon: That's preposterous. I do not resemble C-3PO. Don't get me wrong, I'm flattered. I just don't see it.
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.