Sheldon: You participate in the mass cultural delusion that the sun's apparent position relative to arbitrarily defined constellations at the time of your birth somehow effects your personality.
Sheldon: I'm not crazy. My mother had me tested.
Sheldon: You have about as much chance of going out with Penny as the Hubble Telescope has of finding that at the center of each black hole there's a little man with a flashlight trying to find the circuit breaker.
Penny: Can you help me? I was writing an email and the A key got stuck. Now it's just going "Aaaaaaa."
Sheldon: Coffee is out of the question. When I moved to California, I promised my mother that I wouldn't start doing drugs.
Sheldon: Oh, well, this would be one of those circumstances that people unfamiliar with the law of large numbers would call a coincidence.
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.