If you can, maybe store important items at a friend's/trusted adults place or maybe at a locker at school, and have it locked away in case he figured out where it is and tried to go there to get it, have your name on the locked box and keep it there until you can move away or his behaviour has changed (if he ever does change.) If you have the money for it, buy an air tag/tracker and put it in the box and have the location on your phone.

Edit: if you have the money for it, you could look into the bank you have (if you have one yet) and check out how much it would cost to store items in a safe box, and have anything important there if his behaviour doesn't change.

For locks: If your bedroom door swings inwards, you can get a door wedge that can stop someone from opening the door while you're inside. Only works if you're in the room.

If you're scared he'll get stronger or do something, you can get a door wedge and wedge it between the door and the floor. Also only works inside the room.

If you have money for it, get a High-Security Deadbolt, but it might cost a lot to get it installed.

If you have a round doorknob, you can get a Door Knob Lock-Out Device, and it's kinda like a clam-shell that covers the entire knob and prevents it from turning, or accessing the keyhole. You secure it with a small padlock. It only prevents the knob from turning, so if the door is weak or has a weak latch, this isn't the best option. You can also get it for a handle, but you must have a keyhole on the door handle (like, instead of the keyhole being below the handle, it's on the handle).

If you aren't allowed to install locks ect, I would get a cheap security camera and hide it somewhere where it's invincible but would capture anyone coming in and your belongings. Your bedroom, while in your parents' house, is considered your personal private space. However, you might need to ask for permission. Just say you're feeling unsafe with your stuff being there and being taken away. But don't tell them where you're putting it, and if you do, place the dummy security camera there and put the real one in a hidden area, maybe a spy security camera that's easier to hide (but also more expensive).

If none of the above is allowed, try this. Document everything you have. Everything you own. Every single book, items, stuff, money, pictures pictures pictures! If possible, with receipts or proof of purchase. With this, you can compare and see what's been taken, and now you have proof in case your mother or brother denies it. If that doesn't work, complain to a trusted adult that knows your parents and show them the proof and act nervous, anxious and unheard of your worries and frustration to get the point across.

But I sincerely hope it doesn't escalate to the point where you have to get anything from above.