I need help.

I have a sister that constantly keeps on taking my stuff. Some of the stuff she took include my art supplies, some of my favorite sculptures, and my diary which she treated to blackmail me with. And the thing is that she doesn't even keep them in good condition. And whenever I confront her she either gets mad at me for "annoying" her or she just denies and tells me to leave her alone. She is older than me, and I am the middle child for anyone wondering. Do you guys have any idea what I should do? Thanks for spending your time reading this.