I'm worried about my brother smoking hookah? How unhealthy is this?

Very. Tobacco is bad for you, pure and simple. However, your brother is an adult. He knows it is bad for him, and he has decided to become addicted anyway. You can't stop him, and he will resent your criticizing him, no matter how helpful your intentions. Let it go. It's good that you care, but relationships are more important than being right.