What if your brother stole something that was very important to you and wouldn't give it back?

You’d have to convince him otherwise that it’s not your property. You could do this physically or through trickery or blackmail. You’d have to get revenge in some form. You could steal something that’s important to him and force him into a trade. Or you could seek help if you’re not able to resolve the situation on your own. You could ask a parent to be a judge or call a friend, sibling, or an enforcement authority for help and team up. One of you could distract him while the other searches and retrieves the item. It might be locked away so you might need to break something to gain access. You’d also have to decide if the effort is worth it and what the consequences would be; even though he’s in the wrong, and consider what has happened that has forced him to take this action. Perhaps there’s a good reason that he’s done this, and that he thinks that he’s acting in your best interests in the long run.