How can I take over the POA my now late sister held for my mother?

I am the only surviving child of my mother. My sister just died and was power of attorney for our mother. My sister's husband just had my mother sign 5 blank checks, supposedly for her bills. My mother is in a nursing home here in Ohio. She suffers from dementia, the only real health threat at her 86 year age. I want to become power of attorney as I love my mother and visit her daily at the nursing home since her admission there two months ago. My brother-in-law is reluctant to change me over to power of attorney, along with his sister and children. Do I have the right to be her power of attorney? She lived with them until her admission to the nursing home and they have told me they have lost records for my mother's wishes and other important papers. Please help I am up against all of them alone.