elder sister staying in trunk
i think that she stayed in the trunk willingly because they were told repeatedly that the outside world was only safe if you were inside the car.
she 'escaped' in her own way.
she had followed all the rules set out for her; not leaving until her dogtooth 'fell' out, and then continued to believe that the car was the ultimate escape. she would have had no idea what else could be waiting outside the fence but danger.
the car was all she could hope for.