NEED ADVICE

So I am the oldest of 8 kids. 5 are adopted. Well I have shared a room with both the boy and the girl of a sibling set that we adopted. Both of them like to get into my stuff and ruin it (tear up makeup brushes, cut my dogs hair, put on my makeup, steal my candy, do into my backpack and take stuff, etc.) and steal it. I didn’t mind at first because I thought “ oh they are just a kid maybe it was a niece maybe twice thing” but it happens constantly. The most recent ones are my younger sibling who I am currently sharing a room with went into my backpack and took out glitter then proceeded to cover himself, my dog, and my entire bathroom in glitter. Then tonight he went through my stuff and lost my mouth guard; I have TMJ so I have to sleep with a mouth guard. I just bought this newer/nicer mouth guard 2 days ago so I’m pretty annoyed at this. I need to know. What do I do to get them to stop ruin,taking, destroying, steal, lying , etc. my stuff because I am fed up with it?? Please help.