Your original narration doesn't follow chronology and hence confusing. I understand this way.
A person had a property in a co-perative housing society in Mumbai. He had besides his wife one son and 3 daughters. You are the son. You and your sisters executed a document during the lifetime of your father, a copy of which you have attached, According to the document the 4 of you have reliquished your rights over your father's said property. Your father had nominated your mother as his nominee in the records of the society. After the death of your father your mother claimed the property as his nominee. The Society transferred the property to the name of your mother. Your mother and her 4 children including you were the heirs to the property. But as 4 of you have abandoned the rights, your mother became the sole owner. She can gift the property to her grandson (I think that he is your son).
Now you do not like your mother transferring the property to your son for whatever reason it may be. But you cannot question the transfer because you had abandoned your rights. The only manner in which you can question the gift deed is by disputing your own reliquishment deed on legal grounds. My opinion is that you have to make a try in the court of law. You can try it in the co-operative court.
