There is a deemed partition as your grand father died intestate. It is assumed that the property has been divided between the heirs equally that is between 4 sons and 3 daughters.
The succession act as amended is applicable in the case of a deemed partition
The unmarried daughter gets the equal share with others. she will get 1/7 share in the undivided property. This can be effected through a decree of partition from court or division with mutual understanding and a guardian can be appointed on her behalf as she is mentally ill.
As per 6(3) of the Hindu Succession Act, 1956 (as amended)
Where a Hindu dies after the commencement of the Hindu Succession (Amendment) Act, 2005, his interest in the property of a joint Hindu family governed by the Mitakshara law, shall devolve by testamentary or intestate succession, as the case may be, under this Act and not by survivorship, and the coparcenary property shall be deemed to have been divided as if a partition had taken place
(a) the daughter is allotted the same share as is allotted to a son;
(b) the share of the pre-deceased son or a pre-deceased daughter, as they would have got had they been alive at the time of partition, shall be allotted to the surviving child of such pre-deceased son or of such pre-deceased daughter;
(c) the share of the pre-deceased child of a pre-deceased son or of a pre-deceased daughter, as such child would have got had he or she been alive at the time of the partition, shall be allotted to the child of such pre-deceased child of the pre-deceased son or a pre-deceased daughter, as the case may be.
Section 30 of the Hindu Succession Act, 1956 (as amended) also clearly states that the interest of a male Hindu in a Mitakshara co-parcenary property shall notwithstanding anything contained in the Hindu Succession Act, 1956 (as amended) or in any other law for the time being in force, be deemed to be property capable of being disposed of by him.
the explanation is that the right of a Hindu Mitakshara coparcener shall be deemed to be the share in the property that would have been allotted to him if a partition of the property had taken place immediately before his death, irrespective of whether he was entitled to claim partition or not.
Since your father did not make any WILL his share will be divided between his wife and children equally if your mother is alive there will be four shares or else three shares
If at all