Sale of ancestral property on j.v get one flat and cash

To decide about the capital gains you should refer to the agreement.

The sale consideration will be the flat as well the extra amount.(the flat part can be assumed to be invested u/s 54F)

Sale of immovable property by way of cash will attract heavy penalties u/s 271D, which is a sum equal to 100 per cent of the amount received towards consideration. This rule applies with effect from June 1, 2015. I suggest you ask the builder to pay the consideration by cheque only. You can then invest in REC bondsupto Rs 50,00,000/-