If you don't share a room with him then putting a lock on your door isn't harsh. You have every right to keep your own things safe, but don't be surprised if your parents insist on keeping a copy of the key. We parents are really annoying that way. ;)

Your brother is having some difficulties growing up and finding his own identity. He therefore tries to take part of yours. My guess is that he sees himself as treated unfairly and justifies his stealing as gaining what should have rightly been given to him. If your parents are not helping to address this, your brother will continue to have this sense of entitlement, and unfortunately such behavior later in life can lead to fines, jail or worse. If I'm right, that he feels justified because he feels victimized, then this is a torturous way to go through life. So for you to deal with this would be to try an experiment. Make a deal with your brother that you will go out with him at least 2-3 times per week to spend time, make him feel cared for, help him learn how to interact with his peers effectively, find the things he is good at, and generally do the job that your parents have fallen down on. Since your goal is to prevent his stealing, help him know how big a wedge this draws between you, but do not punish him by ceasing these outings. Instead, when he's been respectful, make the outings extra special. If he's invaded your room inappropriately, then simply go for a walk and talk with him about life. By the way, it's OK to include him in outings with your friends, so long as he knows you're not going to allow him to be picked on (until he can adequately defend himself).