I know someone is stealing from us

Long story short:

. We have had people go into our room when we aren’t home or in the showers and take our stuff! We suspected it was his friend because his friend lives in a back house as well and his siblings partner. We caught his friend red handed but he had stopped a while back and stopped being friends with him. His sibling and partner just moved back in. I’ve had more than 500 stolen from my room a few weeks ago and many clothes and items taken. I’m going crazy living here because there is no respect for MY belongings I worked hard for and for my privacy. an hour ago me and my fiancé went to wash all our animals and dogs and when we got back into the room we noticed right away someone had gone in! After I washed my goat and dog, I put them inside the room and locked the door from the outside. I went back inside to wash my last dog and when I got back, my other dog was outside and my goat was still inside with the door locked shut but the handle was sideways. My fiancé had a couple nugs of weed stolen and that’s the only “damage” we seen so far but very very obvious someone was here by the way the door was, the dog outside, and the weed noticeably touched. Mind you, his siblings partner was inside and she disappeared for a bit then popped back up. I am soooo suspicious of this person. And when me and my fiancé were talking bout it, we heard his sibling and partner snickering. What should I do!? They don’t stop