My mother has always said - things need to be fair for my three daughters. It seems the middle sister wants all the control. How do we proceed?

My two sisters and I own a commercial building worth $1.2 million. Only my mother and my middle sister on the checking account attached to this property. My youngest sister and I have asked to be listed as signatories (on the checking account); however, the middle sister is balking and saying, no, you can't be on it. My mother has always said - things need to be fair for my three daughters. We are at a loss of how to deal with this. It seems the middle sister wants all the control. How do we proceed? Do we get a lawyer? We are worried that if my mother passes away (or my sister), neither of us will have access to the account to pay bills, or inherit.