The Work Song Nanocluster - S2-E18
Trivia: After Sheldon mentions a molecular sieve, they go over to 4a, their apartment. 4a is a kind of molecular sieve.
The Bath Item Gift Hypothesis - S2-E11
Trivia: Kaley Cuoco's favorite moment of Penny's is when Sheldon hugs her after receiving the napkin signed by Leonard Nimoy.
The Work Song Nanocluster - S2-E18
Trivia: Penny is the only character whose original surname was never officially revealed (she took Leonard's surname when she married him). That said, a close zoom in on a package she receives in season 2, episode 18, "The Work Song Nanocluster" appears to show her surname as "Teller". The creators have said that was simply a default placeholder prop and was never meant to be legible on screen, so it's not canonically her surname.
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.