The Long Distance Dissonance - S10-E24
Continuity mistake: After the 'Ramona' conversation between Sheldon and Penny ends, Sheldon leaves Penny alone on the couch. Penny looks up to the "Amy and Penny" painting and says "Don't look at me like that, I tried." In S10 E10, Amy gave the painting back to Penny, which was hung in Leonard's living room.
The Long Distance Dissonance - S10-E24
Continuity mistake: When Sheldon takes a plane to see Amy in Princeton, the plane he is on has 2 engines when it takes off and 4 engines when it lands. It's a direct flight from LAX so he wouldn't have changed planes.
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.