This is a simple situation. Your dad owns the vehicle, so he can either take her to small claims court and get a civil judgement to have the property returned... or he can bypass small claims and have the sheriff help you reclaim the property.
My dad had to repo a car that was fraudulently financed by his dying mother for her caretaker (elder abuse)... he simply marched into their garage and said "I'm here for the car. Give me the keys NOW." They handed over the keys... and he drove it away. Simple...
Just don't get shot doing it.
