What do you when someone fills out a form naming another sibling as Healthcare POA when they are not - just because they are angry.

I have two sisters that are so very angry with me and continue to harass and bully me. We needed to get a sitting for Mom since she has fallen several times and the financial POA refused to decide on which company to use. She refuses to let the other siblings know mom's finances - what mom can or cannot afford. The memory care facility had two recommendations and it was left to me to choose without know how much mom can afford. I found the contract that was signed and saw the contact list showing that a sibling was named healthcare POA who is not. Is it illegal to do this? This person is not allowed to make any medical decisions about mom. Mom is in the last stages of Alzheimer's and I am concerned that if something happens to mom - the rest of the family won't be notified.