Wolfear, have we got the same Mil???
I had to give Dp an ultimatum, change the locks or I move out.
Mil used to let herself in without knocking before or anything, then look for me inside the house, including in our bedroom, or in the bathroom. I have stopped playing the guitar, as I couldn't hear her coming in. She used to frighten the life out of me.
When I was at work, she used to come in and rummage through our mail, our drawers etc. She got caught reading Dp's bank statement (so concentrated on it, she didn't hear ME come in), then stuttered that she thought it was her husband's. WTF? She also changed all my kitchen cupboards contents around, saying it was more convenient her way. I have found her sitting on the edge of my bed as I lay asleep, about a week after DD1 was born.
As for Fil, he has come in the bathroom while I was having a shower, looking for me.
It came to blows as I couldn't sleep anymore, wouldn't put the music on anymore, wouldn't walk around in my undies, would get undressed superquickly in my room for fear they should walk in. I was a paranoid wreck.(they used to "pop in" 3, 4 times a day!!
Dp changed the locks, the ils demanded a spare for months, we held on, and I have now learnt to live again (iyswim).
We only give them a key when we go away on holiday, and that's bad enough. Last time, Mil found a tiny Disney toy underneath DD1's mattress (as small as half of my little finger). DD1 sleeps in a wooden elevated bed, so to get there, you'd have to climb up the ladder and run your hands along the edges underneath the mattress. .
She also gave me a book of mine, proudly announcing that she found it deep underneath my bed, "you must have been looking for that for a long time!".
Sorry for the essay, OP, you've touched a raw nerve...
To me, YAdefNBU.