I had this situation too when I was younger. Only way to resolve it was to get a lock, and lock the door each and every time I left the room. Even if I went to the toilet or something..
my sibling would take any opportunity they got to slip in there unnoticed, and once or twice when I left the room for a short period of time and "forgot" to lock the door, I payed the price ...and stuff went missing.
Literally the only way to ensure stuff didn't go missing was to get a lock.
and get a bump key resistant lock too. I don't think mine was bump proof, ...and tbh I think if he knew about bump key's he would have ordered one off Amazon or someplace just to get in there,