Well, you can only know so much about a person as they're willing to let on. The fact that she, despite being his sister, is surprised by the amount of followers he has proves that point to some extent.
Many people keep quiet about their problems and concerns, choosing to put on a facade that everything's fine for various reasons. It can have some serious consequences, since if noone but you knows what's going on, there's the risk of others not realizing until it's too late how serious the situation really was.