How can I get my sisters to believe that our mom has been diagnosed with mild to moderate dementia?

RLP gave excellent advice. It's sad that so many sibling relationships fall apart over elder care, but it happens often. If your sisters can't be kinder about it, all you can do is tell your mother that you love them but they and you disagree about some things. Tell her you are working on making it a better relationship (when she presses). Otherwise, as RLP said, tell her you love her and that all you want is for her to be healthy and happy. The doctor and attorney should be contacted soon before your mother becomes more impaired. Good luck. We know this is tough.
Carol

Can you have your doctor write up his/her diagnosis and have it sent from the office to your sisters? Next, can you take a copy of the diagnosis to an advocate or attorney for the elderly? Those two actions might result in some restrictions being placed on your mother's accounts, etc. Also, you might be able to start getting some services such as a part-time sitter for your mother. I have found that my mother, who also wants me to be close to my sister, responds when I tell her that I just want her to be well and safe and that I love her. It makes her smile and forget the sadness. Best wishes to you and your mother. P.S. (Start documenting everything in a notebook.)