Sister can’t stop stealing things during visits

DEAR ABBY: I’m having a problem with my sister, who visits me on the average of once a month. She lives four hours away. Her friend and her husband accompany her. They are my only company. I used to enjoy them but no longer. After their visits, stuff is always missing from my house — a hairbrush, my dog’s electric nail file, random dishes and plates, rat traps, laundry detergent packets, dollar store craft items, etc.