I hate everyone saying not to Palm it off on the girl. If she understood boundaries she'd know it's weird to take someone else's things without their okay and would probably have brought them back immediately. So either the brother lied to everyone including her about it or she has very little respect for boundaries.

Op get a lock box and prepare all of your stuff for when you go away. Store all of your clothes in bins and in a place only you would know about. Keep a small log or note of everything you're storing so you can keep track of it. Better anyway, in case you have valuables that go missing, so if you have to file a police report, for whatever reason, you know exactly what is missing and the value of it.

You're almost 30, you should be preparing not to live in your house anyway and you barely live there now. I get that you expect it to be a place where your crap is safe but it's your parents house, where it's a shared living space, not a secure storage unit.