The Holographic Excitation - S6-E5
Trivia: In this episode, Sheldon and Amy are trying to decide on couples' themed Halloween costumes. Sheldon is standing in front of the dry erase board. There are 2 columns written on the board. One named "Couples I Like" and the other "Couples You Like." Under the "Couples You Like" column, one of the couples is Blossom and Joey. Blossom is the name of the television character played by Mayim Bialik in the 90s and Joey was one of her brothers.
The Fish Guts Displacement - S6-E10
Trivia: In the beginning, after talking about the Spider-Man theme, Sheldon says it's right up there after Inspector Gadget and Teenage Mutant Ninja Turtles (to which the gang chimes in "heroes in a half shell, turtle power"). Chuck Lorre, the show's creator, wrote the original TMNT theme song that they are referring to.
The Holographic Excitation - S6-E5
Trivia: On the couples costume idea board, "Dharma and Greg" is listed. Chuck Lorre, creator of The Big Bang Theory, also created Dharma and Greg.
Trivia: One of Penny's classmates, Cole, is portrayed by Ryan Cartwright. Ryan Cartwright is a main character on Alphas. At the time of the episode, Alphas hasn't been cancelled. Episode 21, The Closure Alternative, deals with Sheldon getting over learning Alphas had been cancelled.
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.