When is it important to get a power of attorney for an aging parent in a nursing home?

She is 85 years old and only receives Social Security pension. My sister has applied for Medical for her and she also used to claim her as a dependent. Mother entered the hospital late December, 2010 and from there was transferred to a nursing home. My mother had a property and this sister and sister's husband hired an attorney so they could end up with Mom's property and they did it very secretly which was not cool. My other sister and I think Mom needs a power of attorney so that one sister does not keep taking advantage of Mom as we do not believe she has Mom's best interests in mind. Also if Mom is in a nursing home is she still considered their dependent for tax purposes?