She's incapable of understanding what graphic design is, and the fact that it's a legit job. I'm not sure if it's a language issue, age issue, or both, but no matter how often, how simply, and how slowly it's explained, it all goes in one ear and out the other. ...DAMN that was fast!!

Because at the time, she was trusted enough to have it.

For a lot of people this would be a YDI for letting the mother-in-law have a key.. However OP has said they thought she could be trusted to have a key. My mum has a key to my house, she lives miles away though. The worst she does is clean the kitchen and bathroom with bleach.

Ours actually wasn't given a key. She nicked my husband's set and made herself copies. With the new house, I installed locks with keys that can only be copied by the original locksmith. The request has to be in writing and signed by either myself or my husband. It will cost a lot more to copy a key if I need to, but it's well worth it. I also installed a security system with frequently changing access codes. Even if she manages to break in, the cops will pick her up in 5 minutes for unlawful entry. Harsh, but with someone who refuses to recognize their son and daughter-in-law's privacy and display some basic manners, those measures became necessary.