oh you poor thing! that is absolutely ridiculous! You are a fabulous parent, and I'm sure your mother in law is just as great.
People are so uptight nowadays! my parents used to leave us in the car when they ran into the supermarket without even being watched. They just said lock the doors, i'll be back in a few minutes. Nothing every happened and they were fabulous parents. It saved them having to go in with us begging for everything in sight. Times have definitly changed, and I'm not sure that its a good thing. Leaving a kid in a car for a few minutes is not a sign of an abusive parent. I'm sorry you have to go through this.
Are they reporting both you guys and your mother in law to social services?