After the commencement of Hindu succession Act, 1056, the property of a Hindu male dying intestate devolves upon the legal heirs as per provisions of Act. Firstly it devolves upon class I legal heirs who are sons, daughters, widow and mother. In your case the property was inherited by three daughters and a son getting one share each, considering that your grandmother did not survive by your grandfather. Since two of the legal heirs, including your mother have relinquished their share in favour of their brother he becomes owner of 3/4 of the estate. I may add that the released deed executed by your mother and aunt has to be registered as per the Registration Act and without registration it has no value. The position was quite different before the introduction of the HSA.