The Classified Materials Turbulence - S2-E22
Continuity mistake: In the comic book store, when Stuart and Leonard are talking about Penny saying "Leonard" when with Stuart, Stuart's pen/hand is constantly changing position instantly.
The Classified Materials Turbulence - S2-E22
Continuity mistake: Leonard is talking to Stuart at the door to his (Leonard's) apartment. Howard then comes over and slams the door in Stuart's face and Leonard has moved back about 4-ft instantly.
The Classified Materials Turbulence - S2-E22
Continuity mistake: When Raj is going through the ideas on what would work to fix the toilet, Sheldon's hand is not near his face, but when Howard talks about moving to Israel, Sheldon's hand is up by his face.
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.