The Escape Hatch Identification - S10-E18
Continuity mistake: Stewart eats a ham sandwich when he and Raj are talking alone in Howard's and Bernadette's living room. On at least one occasion the sandwich's size gets larger, not smaller between bites as Stewart is eating 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.