The property has so far not been transferred to anyone's name though there exists a GPA on your grandmother's name who has again executed a GPA on your father's name.
Firstly, though your grandmother physically acquired the property she has not obtained a marketable title to the property. The only valid document was a registered GPA on her name, this document will not confirm her the marketable title until she executed a sale deed unto herself in the capacity of the GPA (vendor - by herself) to the buyer (herself), also the unregistered sale agreement is not valid.
Secondly, being a GPA herself, she cannot execute a GPA on the same property to another person(your father), hence the GPA on the name of your father is invalid in the eyes of law.
Thirdly, a relinquishment deed by your mother and sister in your favor after the demise of your father assuming to have rights to the property in which even your father had no title, was unnecessary and invalid in law, by this you cannot claim title to the said property.
Therefore you have to locate the original owner or in case he is dead then his legal heirs, convince them of the situation and execute a proper sale deed registered in your favor by paying the proper stamp duty applicable on this date and acquire marketable title to the property.