Worrying about him won't help whatever is wrong with him. But you don't need to fake smile either. Ask your parents or even his doctor if there is anything you can do to help him when he's released from the ICU.
If you aren't worried or upset don't pretend to be, just don't be mean or in-sensative. Just do whatever you think is right don't pretend you feel one way if you feel another. btw sorry if I sound mean or anything, but why is ur brother in intensive care. (If you don't want to say you don't have to, you could always inbox me otherwise if you don't want everyone to see. Anyway if you can't say I'd understand)
Be realistic about it, but not too realistic. Be as optimistic as u can about it. Having an optimistic attitude will help u be happier,and there's still hope for him. And also, if u feel like crying, then cry. U sometimes just need to let it out. :) I hope this helps.