The Discovery Dissipation - S7-E10
Continuity mistake: The Stevenson Award in Sheldon's office reads "Sheldon Cooper, PhD." But, in the "Dennis Kim" episode we are told Sheldon was 14.5 years old when he earned that award - 1.5 years before earning his PhD at age 16, so the award should not show the title of PhD.
The Discovery Dissipation - S7-E10
Continuity mistake: In Season 3 Episode 1 Sheldon made a retraction of his paper. But here he says he had only one retraction, when he was a kid. (00:11:36 - 00:13:09)
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.