Firstly, does she have a witness? The bus driver is unlikely to even know he has hit your sister's car. It is imperative you always take photos at the scene. Contact the bus company directly and ask for their insurance details, advising that she finds them wholly liable for the damage. She is giving them the opportunity to resolve the situation. Next, go to more than one bodyshop, not a mechanic, and ask them to evidence what is needed to repair the car correctly. Create a pro forma to establish the value of your loss. You then have an idea what it will cost.

Is the vehicle mobile? If not, advise the bus company you need an alternative vehicle. If they cannot arrange this, you will have no other alternative than to use a claims management company. As a last alternative, use your insurer. But advise them that, should the premium increase, she will wish for them to claim back the cost of this increase for the next five years from the at fault party.