ALWAYS accused of stealing from sibling?

Okay, so my younger sister has a bit of an attitude and small brain and I'm a bit of a pushover but I don't steal or lie. When she loses her belongings, she automatically assumes i've stolen it from her. So with convincing herself that i've taken it from her room, she makes nasty comments and makes me feel like i've actually have done something wrong. I stand my ground and tell her she should look harder instead of accusing me everytime.This happeneds almost everyother day, and leads to fights. She's a bit of a sour person and a bully and gets me stressed out trying to defend myself and then she walks away smug about it and makes comments when i walk away. When I'm not home, she goes into my room and takes things from me (thinking ive done the same thing to her) and when iI'm trying to find these specific things (and confront her, and she plays dumb at first, but then after 10 minutes of going mad trying to find my stuff, i ask her again and she goes "oh, you mean this?" and hands it over nonchalantly. THIS DRIVES ME CRAZY. How can i possibly asses my stress levels when i am constantly being wrongfully accused of theft and then being a victim of theft by the accuser?? I'm sure this happens all the time with siblings (sisters mostly) but I'm having trouble keeping my cool trying to be the mature person in this without losing my sanity. Any advice? (my younger sister is 18 yrs old, I'm 21)