my sister has power of attorney for my parents. What happens if something happens to her? Can all 3 sisters have power of attorney?

It is up to your parents to decide to whom they grant a Durable Power of Attorney and to name successor agents if the person they name is unable or unwilling to act.  While some lawyers require that an agent under a Durable Power of Attorney provide an accounting to the siblings or others to prevent suspicion, others think it will lead to discord as those who are not the agent may go over it with a fine tooth comb and question the agent's decisions.  Some lawyers advise have co-agents (two, not three) for major decisions such as liquidating investments or selling the house, with the co-agents to act together.  Appointing three co-agents increases the chance of disagreement and failure to act timely if they must act together and of "the right hand not knowing what the left hand is doing" if they do not.  It is generally not recommended for the day to day responsibilities which you describe.