The Spaghetti Catalyst - S3-E20
Continuity mistake: When Raj and Leonard are sitting on the couch, Leonard is holding Watchmen and Lost in Space in his left hand and Young Sherlock Holmes in his right. In the very next shot when Leonard turns his head after Raj speaks, Watchmen has disappeared.
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.