Income tax treatment on sale of property at mumbai

Hi,

let me state the details of the case.i am using "me" and "mine" where appropriate.

A building with 3 flats was purchased by my uncle in 1965. One flat was given to my parents and the remaining two was for my uncle.[my parents had given money in return for the flat, which does not exist in any document, though]. In 2015, the building went for reconstruction.My parents got 2 flats in the new 10-storey building in lieu of the flat they surrendered. The new building is under construction and will be completed by december 2016. My parents now want to sell both the flats in the new 10-storey building. 

How will the income tax be computed? What will be the tax treatment for this sale?

thanks