Problem with lawyers

As per your statement, you, your mother and your sister received 1/3rd share of your father's property by virtue of a decree passed by the civil court. Now your mother's 1/3rd share devolves on yourself and your sister, for which you can arrange a settlement between yourself or if its not possible to settle it amicable, another partition suit has to be instituted by either parties with respect to the share of your mother. The decree passed is with respect to your father's property and you may have to file a suit for partition between yourself and your sister with regard to your mother's property. 

Did your advocate say anything about amending the decree passed by the civil court ?. I dont think the court will allow amendment since the death of your mother is after the passing of the decree so the only way in my opinion is to file another suit. 

As far as your query with regard to changing your advocate, if you are not satisfied with his services, you can ask for a change of vakalat, but that cannot be done without his intervention as he has to give consent to permit you to prosecute the case through a different counsel.