The Work Song Nanocluster - S2-E18
Continuity mistake: When Penny puts the box down at the start it is closed. Camera cuts and it's open instantly.
The Work Song Nanocluster - S2-E18
Continuity mistake: When Penny gets the package, the position of the box on the counter keeps changing position.
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.