I'm in quite a complicated situation with my landlord at the moment. I live in a shared house with her and another friend. The house belongs to her parents and it's her first time living away from home. They are used to renting properties and she's never done it before and acts much more like a tenant of theirs than our landlord most of the time. Because of this whenever we have queries about rent or maintenance matters we have been given contact details for her parents and go to them.
Apart from previous grievances over never turning the heating on and her not pulling her weight with the housework, the current issue concerns my privacy.
Me and my other housemate went on a weekend away a couple of weeks ago. We knew that she was having a first date that weekend and inviting him round. I had a niggling thought in the back of my head that if she wanted to shag this guy (a whole other issue), she would use my bed (her parents wanted to maximise the rent so she has the box room with a high sleeper whereas I rent the master bedroom). I therefore arranged a few items on my freshly made bed and took a photo for future reference. At the time, I felt incredibly guilty about even thinking that of her. 
When we got back I noticed as soon as I walked in that something wasn't right. Despite her careful replacing of each and every item, I could tell that every part of my bed had been disturbed, especially when cross referenced with my photo. Even the bedside lamp!
And upon closer inspection there was a mystery stain in the middle of the bed! 
The next morning I told her very firmly that she needed to wash my sheets, she tried to play it off at first, but after realising my anger she did this.
The problem now is that she mentioned in passing that she's holding a new years eve party in the house. Me and my other housemate are spending the holidays away with our families.
My gut tells me the only way I will be sure that my privacy is maintained is if I install a lock on my door, however to do this I would have to screw into the wood which requires permission from her mother. I know that she will ask why I want a lock and to explain the situation to her feels very much like childish telling tales.
My other option is to sit my landlord down and tell her firmly that it is unacceptable for her or anyone else to go into my room. But how do I make sure that when I come back in the new year I know if she's been in my room?
I've lost my peace of mind and could really do with some advice! 