The Spoiler Alert Segmentation - S6-E15
Revealing mistake: Throughout the series much is made of the fact that Howard's mother is grossly obese, and in The Engagement Reaction (S4 E23) it is even mentioned that her arms are hugely oversized. However, at the end of this episode we see her arms when she grabs Raj to stop him climbing out of the window, and they are the arms of an average-weight woman - they aren't overweight at all.
The Spoiler Alert Segmentation - S6-E15
Other mistake: When Leonard and Sheldon are about to make up, you see green lights flash in the hallway. The next shot you see the hallway and it's completely normal.
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.