Do not worry as taking documents relating to the proeprty does not make your uncle sole owner of the all the properties left by his father.
On death of your grand father without any Will all of his children will be entitled to equal shares in the properties left by him.
So your mother has 1/3rd sahre in the proeprty of her father provided her mother is dead.
Since your mother mentally unstable apply for appointment of your self as his legal guardian and then file a suit for partition to claim her 1/3rd share in the proeprty. in the said suit claim injunction as well.