Siblings disagree over selling parent's house
Since your sister has power of attorney, she can make financial decisions on your mom’s behalf. To sell your mom's house, the power of attorney document your mother signed needs to be all-encompassing, or it should specifically state that your sister has the power to sell your mother's real estate assets.
If you can't convince your mother to revoke the power of attorney, the only way to challenge your sister's authority is to go to court and petition to be your mother's guardian or conservator. This is a time consuming and expensive process. You would also have to demonstrate that your sister is not acting in the capacity to which she was appointed.
If your sister has poa, is she paying bills for your mother?. If your mother is in a nursing home perhaps they are requiring more money than your mother has and therefore your sister may need to sell the home
You need to find out what type of POA your sister has there are 2 types of POA. The Durable POA which gives the person the right to sell property (complete authority to all financial issue). I can't remember what the other one is call. You also need to ask your sister if the up keep of the house is getting to be a financial burden. This might be the case.
