I'm living it, leaving it to change
idk, I get how annoying that is because my older sister used to steal my expensive makeup. I'd look through her makeup bag every couple of weeks and have to dig all my stuff out. I've also had her break stuff that she refused to replace once she looked the price up online and realized it was more expensive than she thought (which to me is even more reason to replace it, but whatev). She moved out years ago, but now I have a younger sister who uses/steals my stuff.
It's just typical sibling stuff to me. Some people are just disrespectful of other people's things, which is funny because they usually flip out if you disrespect theirs in return. Stealing from him is only going to stir up drama, probably. Just hide your stuff or put a lock on your bedroom door or something. People like that don't really change.