The Thanksgiving Decoupling - S7-E9
Continuity mistake: When Sheldon and Mike are drunk on the couch after Howard leaves, they are toasting with the beer cans. Sheldon has his in his right hand, but when Bernardette came in, the can is in his left. A little bit later, after he says "she's so tiny", the can moves back to his right hand.
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.