Hello Sir,
For an ancestral property, the period of holding is reckoned from the date of purchase of property by the owner who actually acquired the property otherwise than by inheritance or gift. As you propose to sell an ancestral property purchased in earlier years (assuming that the property was acquired by the original owner otherwise than by inheritance or gift), the resulting gains shall be classified as LTCG and you can avail indexation benefit.
In case of an inheritance, the cost of acquisition should be the cost for which the previous owner who has actually acquired the property otherwise than by inheritance or gift, as increased by cost of improvement made subsequently. Since the property has been acquired by the original owner prior to 1 April 1981, you have an option of taking the actual cost of acquisition if available or the fair market value of the property as on 1 April 1981.
You can contact me for further clarification.