The Vegas Renormalization - S2-E21
Continuity mistake: When Leonard and Raj are getting ready in the bathroom, Raj picks up a tube of shampoo/conditioner off the bathroom bench with his left hand. However, in the next shot, Raj has nothing in his left hand and the tube is back on the bench, with no time for him to replace it between shots.
The Vegas Renormalization - S2-E21
Continuity mistake: In the bathroom of the hotel in Las Vegas, we can see Rajesh taking a small white bottle and keeping it in his hand while saying "I love Las Vegas". In the next shot, the bottle has returned to its place instantly.
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.