The Good Guy Fluctuation - S5-E7
Continuity mistake: Leonard refuses Penny's offer of a glass of wine because the sulphates in wine give him migraines. This makes no sense - we have seen him drinking wine in too many episodes to count without him once complaining of an adverse reaction.
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.