The Werewolf Transformation - S5-E18
Continuity mistake: When Penny is giving Sheldon his haircut, she accidentally shaves the back of his head. The clippers are going from the bottom of Sheldon's hair, up to the top of his head. When Sheldon goes to leave the room, it can be seen only four or five inches from the back of his head have been shaved off.
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.