I understand that you think you have every right to be in my home, since my MIL (without my permission) told you you could live with us for free, but you need to seriously start respecting our house. What's that? You don't think it's a big deal
that you left a window open during a rainstorm and soaked one of our rooms, which I had to clean up? You don't think it's a big deal that you took my lunch to work for yourself? You don't think it's a big deal that you spilt sugar all over the
counter and then left it for me to clean up? You don't think it's a big deal that you sit on our new couches in your work pants that are covered in grease? Oh and what's this - most of all you don't think it's fair for us to ask for rent from you?
I would start letting them know that since you didn't invite them yourself that the situation is not working for you - the homeowner. Being passive aggressive as I am I would start making things really uncomfortable and asking repeatedly