Inheritance of grand father's property

It is believed that your grandfather was Hindu and has deceased and your father was properly adopted and it can be proved.

There are rules for declaring a person mentally unsound and you may have to obtain it from o/o Civil Surgeon or medical college.

It shall be proper to consult local lawyer dealing in such/property/revenue/civil matters and your lawyer may opine to partition and mutate the properly properly, before you proceed further.

You have not stated your religion.  Assuming that you are Hindus, on the death of your GF, the property will be divided equally to Daughter of GF and your father.  If your Aunt is ready to give her property she can directly gift her property to you both through registered gift deed relating to her half share. 

Relating to your father's property, since he is mentally ill you have to file a civil case for mandatory injunction declaring him as mentally ill person. The court will refer for examination of your father to Government Doctor. Basing upon the report of doctor, the court will decide whether he is mentally ill or not.  Suppose if he is declared as mentally ill / lunatic person, his part of property will be kept with official liquidator and after his death the property will come to you by way of succession.  If you can convince the court to pay atleast the interest amount to you, the court may grant it. 

Thanks everyone for your reply. We are planning to take it legally by going to the court and applying for a civil case regarding my father's mental health. Though we have all medical history of him for the past 20 years, we have decided to go in the proper channel. Thank you very much.

In addition to experts opinion;  Upon getting declared your father as mentally ill person through a mental health original petition, one of his children or his wife may file a petition to appoint a court guardian  or declare themselves as his guardian to take care of his movable as well as immovable properties including any share of property to be inherited etc.  Now, you have to confirm that your father was legally adopted by your grandfather, on confirmation, your grandfather's original daughter may, if she desires to give away her share of property too to your father, she may execute a registered release deed relinquishing all her rights in favor of your father.  If your father is not a legally adopted son to your grandfather, the intestate property of your grandfather will devolve on your grandfather's daughter, which  she can transfer it on your name or the person of her choice by executing a registered gift deed. consult a local lawyer with all related papers/documents, take his opinion on further issues  and proceed.