Throughout The Big Bang Theory, Sheldon had inconvenienced, demeaned, and hurt his friends over the most mundane things. Given this, it's curious why they still decided to not just tolerate him, but stay friends with him. It was Bernadette who actually explained why. According to her, she and the rest of the Pasadena gang have been quick to forgive Sheldon for his transgressions because he doesn't know what he's doing. "Sheldon doesn't know when he's being mean because the part of his brain that should know is getting a wedgie from the rest of his brain," she says.
This isn't really accurate, and the shorter-haired Missy points that out. Missy goes deeper in her character analysis of her brother, claiming that he uses his social struggles to always try to get what he wants. Sheldon surprisingly agrees with this assumption, essentially debunking Bernadette's explanation of his bad behavior.
Missy recognizing her brother's ploy is impressive, but it shouldn't be surprising. Young Sheldon has proven that she is the most intuitive in her family. With having to deal with Sheldon being her twin for years, and it makes sense that Missy understands her brother.