My brother just got me a new computer but he is always on so I can't play his steam shared games. Is there a way to play his games while he is playing a game too?
Like the comments to your question indicate, Steam Family Sharing is working as designed. It doesn't let you play games on another person's account if that person (or someone else he's sharing games with) is already playing a game on that account. Only one person can be playing a game on a person's account at a time.
If you want to play one of his games you'll either need to wait until he's not playing or buy your own copy of the game for you own account.