My Grandfather and great aunt owned a home equally. He left it to my grandmother in his will. My mother was power of attorney at the time of my aunts

It is very important to look at the deed that established the joint ownership in grandfather and great aunt.  If they owned it with right of survivorship, the last to die owned it all at the time of their passing, and that person's will controls who inherits it.  In the absence of a will, the state statute determines who inherits from that person.  On the other hand, if they owned the property as tenants in common, each owned an undivided one-half interest and each passed that share to their own beneficiaries through their own wills (or state stature in the absence of wills). Powers of attorney normally expire with the death of the principal who signed it.