If the family settlement records transfer of property from name of one person to another then the same would attract stamp duty. If the properties had been acquired from joint family funds then the stamp duty can be minimized on that account.
The law of succession to property would depend on your religion and the same would be governed by the concerned law of intestate succession. In the case of Hindus, the children of the sister will continue to have a share in the property even after her death.
Since your uncle is no longer alive, the legal heirs of your uncle will have to execute a tranfer deed / conveyance of the property in favour of your father and such document will have to be registered with the Sub-Registrar of Assurances.
Real Estate (Regulation and Development) Act 2016' makes it mandatory for all under construction projects as well as completed but Occupation / Completion certificate is yet to be received to register under the said Act and upload full information about the project whereby anyone can check all approvals, titles etc. online.