They are taking advantage of you. I'm so sorry- I've been in this position, many of times. It sucks. Your employers know they have so much power over you because not only are they your bosses, but they're letting you live in their house rent free. It's a power struggle, and they always win.
Do yourself a favor, sit down and rethink your budget- see if you can afford to rent a room somewhere and find a new live-out job. That's what I did after two years of going family to family dealing with this BS. I never thought I could afford it, but I made myself. Now I'm happier than ever not living under someone's constant speculation and being at their beck-and-call. I can come and go as I please and don't owe them anything besides rent.

I know quitting your job and moving out is way easier said than done, but I truly hope things work out for you.