Hi
if you have a will written by your uncle for the properties held by him in USA, then you should get the will probated in USA only.
You will need the copy of his death certificate, the Will(Original), the residence address, proof of identity of your mother and her brother to initiate proceedings.
We can help you contacting the lawyer in USA and also help you in completing the formalities pertaining to probate of will and transfer of properties in your mother's name thereof.
Your mother will be required to attend the court in USA three times for giving evidence.