My sister drove my brother and I to school today. We parked on the far side of the lot were there were no cars and pulled through to an open space. I opened the back right side door to get out and a car pulls in next to us causing my door to catch on his driver's side mirror. My door bends back almost parallel to the passenger door. The hinge is obviously broken and the door won't shut all the way without a full body slam into it, while the other guy received a nasty scratch on the back of his mirror. We exchanged numbers, but I'm wondering how much each of us may be at fault?

Your sister should notify her insurance carrier ASAP.

You and the other car's driver were each at fault. I am not sure, however, whether your sister would be held liable for your carelessness. How old are you?

If your car was stopped and the other driver was moving I'd put the fault on him. Like AK said, report this to your insurance company and it should have been reported to the police.

I go the other way, passenger has a obligation to look before opening door. She opened door into the other cars path, I put the passenger at fault.
So the driver could sue the owner of the car, driver of the car ( who is to have control over passengers) and the person who opened the door.

But yes turn it over to insurance and let them handle it.

I'm with Chuck -- the person who opened the car door should have been watchful.

Yes, call the insurance company and let the two companies hash it out.

... sue the ... driver of the car ( who is to have control over passengers) ...

Where does that (driver must control passengers) come from? I don't believe driver is strictly liable for the actions of the passengers (unless we are talking about imputed liablility of parents for negligent acts of their children).

I wasn't there so I can only guess the timing of this but the driver of the car who did the hitting was not in control of his vehicle and failed to avoid an accident. I can't picture the door as the one that did the hitting, the car was moving and hit a parked car. Insurance company can settle this anyway.