The Hawking Excitation - S5-E21
Character mistake: The sign on Howard's laboratory door reads "RESTICTED area".
The Hawking Excitation - S5-E21
Continuity mistake: When Sheldon is trying to convince Howard to let him meet Stephen Hawking, Howard places a water bottle on the table he is sitting in front of. In the next shot, it appears in his hand again.
The Hawking Excitation - S5-E21
Continuity mistake: When Howard and Bernadette are in his bedroom, there is a bottle of Vitamin Water to Howard's left. In one of the shots, the bottle has completely vanished out of frame, but returns in the next shot.
The Hawking Excitation - S5-E21
Continuity mistake: Penny is talking to Sheldon in the laundry room and places a box of detergent on the machine. The side of the box is facing her. In the next shot, the box has turned to face her.
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.