I'm sorry about your mother's death. The IRS will not release tax information without "letters testamentary", which you can only get by being formally appointed as the administrator of your mother's estate in the Probate Court. You will need to consult a knowledgeable probate attorney for assistance.
The information you obtain at this site is not, nor is it intended to be, legal advice. You should consult an attorney for individual advice regarding your own situation.