The Paintball Scattering - S12-E11
Other mistake: When Penny and Leonard are in the paintball shack, Leonard won't shoot Penny because they are too close and it would hurt. Penny doesn't believe him, but eventually, he shoots her in the chest, and she says in pain, "You're right, we're way too close." But Penny should already know what this feels like; she was shot in the chest from close range in a paintball shack by Sheldon in one of the earlier seasons when she shot his cushion with a paintball. So this feeling shouldn't surprise Penny in S12.
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.