The Panty Piñata Polarization - S2-E7
Continuity mistake: When Leonard talks to Penny about having to live with Sheldon he has his hand up to his chest. Camera swaps to a side view and his hand is not in front of his chest any more. (00:07:30)
The Panty Piñata Polarization - S2-E7
Continuity mistake: At the start when Howard is asking about the house, the pencil in front of him keeps moving on the table. Note the point and the edge of the table. (00:02:30)
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.