I co-own all vehicles with my parents (because we share at times), however unless there is no other way to go around we don't use each other car. If you still have to, we just try to co-op with the rules of the perm driver for that car (like chips and other junk are prohibited in the car I drive, etc.). To me your stepdad shows no respect for you (as well as your aunt/uncle), and I'd just not give him a key for the car.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."