Hi
1) In your case, when the rest of siblings were minors in 1969, and as such the eldest son cannot claim valid title on the property on grounds of mutation when the rest of siblings were minors.
2) Mere Mutation of property in Mandal records(Pahani, Adangal etc) does not confer ownership of the property.
3) In order to claim ownership and title of the property, a settlement deed or a partition deed should have been compulsorily registered under section 17 of Indian registration act by paying registration fees and stamp duties and in this deed all of the family members should have affixed their signatures in presence of two witnesses.
4) Your uncle(eldest son) cannot deny your father or the other siblings their respective share of your grand mother's property on the grounds that he had mutated the property in his name in mandal records.
5) The concept of adverse possession is not applicable amongst family members and hence your uncle(eldest son) cannot claim adverse possession.
6) The Supreme Court in Chatti Konati Rao & Ors. vs Palle Venkata Subba Rao has explained the underlying principles in cases pertaining to claims of Adverse Possession
a) It is a well-settled principle that a party claiming adverse possession must prove that his possession is "nec vi, nec clam, nec precario", that is, peaceful, open and continuous.
b) The possession must be adequate in continuity, in publicity and in extent to show that their possession is adverse to the true owner.
c) It must start with a wrongful disposition of the rightful owner and be actual, visible, exclusive, hostile and continued over the statutory period. (See S.M. Karim v. Bibi Sakina AIR 1964 SC 1254, Parsinni v. Sukhi (1993) 4 SCC 375 and D.N. Venkatarayappa v. State of Karnataka (1997) 7 SCC 567)
7) Also that the daughter's extinguishing their rights by way of notarised stamp papers is not the right method for partition of property.
8) The sisters should have executed a registered settlement deed at Sub-Registrar office as mandated by Section 17 (b) and (c) of Indian Registration act failing which they can continue to stake their claim on the ancestral property.
Section 17(b) and (c) of Indian Registration act reads as follows:
(b) other non-testamentary instruments which purport or operate to create, declare, assign, limit or extinguish, whether in present or in future, any right, title or interest, whether vested or contingent, of the value of one hundred rupees, and upwards, to or in immovable property;
(c) non-testamentary instruments which acknowledge the receipt or payment of any consideration on account of the creation, declaration, assignment, limitation or extinction of any such right, title or interest; and
Hope this information is useful.