Other mistake: Sheldon would not have been able to crawl straight along the window ledge to Leonard's bedroom due to the layout of their apartment. Leonard's window is round a corner from Sheldon's.
The Creepy Candy Coating Corollary - S3-E5
Other mistake: When Sheldon signs the entry sheet Stuart claims he wrote "Die Will Wheaton Die." But he was only writing for half a second to a second, he could never have written all that in the time frame.
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.