I have a brother who is very immature. He always thinks he knows everything and thinks a lot of things very simply. Is there any way to make him mature?

People's maturity can only be realized through many things, and some people will mature early when they start to understand. Since your brother is such a person, you should tolerate him. God gave you such a brother to learn to understand and tolerate. Sooner or later, after he has experienced enough personnel and blows, he will certainly mature. You should have confidence yourself.