Can step-mother control all assets of the estate for her own benefit?

The fact that you indicated that there is an "administratrix" of an estate rather than an "executrix" leads me to believe that your Father dies "intestate" or without a will.  Under the intestacy laws in New York a spouse is given a certain amount of the estate, but children are also given a certain amount. 

That being said you can only inherit what your Father left.  The assets of married people are often considered "joint" assets and some of them - like houses owned as Tenants in the Entirety - assume that there is a "right of survivorship", meaning that the asset passes from one spouse to the other automatically at the time of his death.  Also, your Father's debts need to be paid from his estate.  Here, given that an estate was created, leaves me to believe that there may have been assets that did not pass automatically and needed to be transferred.  You may be entitled to some of those assets that remain after the debt is paid.

As an heir you are also entitled to an accounting by the fiduciary (administratrix).  You shoukd cosider taking all the informaton you have to an attorney in your area and seek legal advice on how to proceed.