What to do if one sister is not consenting to sell our ancestral home?

A property jointly inherited by many siblings more often than not runs into this kind of conflict wherein one or the other sibling makes it difficult for the rest to reap the benefits of the inheritance left by their ancestors.

This becomes all the more difficult and complicated in cases where the inheritance comprises of a joint immovable property like a house / plot of land and the same was passed on to the legal heirs and successors without a legitimate Will. Based on the facts mentioned by you, your case falls under this category apparently.

The best way forward in your case would be to find a way through which you and your brother along with the 2 willing sisters can talk a way out with the unwilling sister as it is always advisable to settle family feuds through amicable methods like mediation etc. as any other legal remedy would not only be financially taxing, but would also be time consuming for all the parties involved.

In case all amicable methods have been tried by you and have not yielded the desired result, the legal way out is by filing a suit for partition and mandatory and permanent injunction before the competent court. You shall require all the documents relating to the property. All the parties who have a share in the said ancestral property shall have to be made a party to the suit. Once the partition decree is passed by the Court and the said property is partitioned by metes and bounds, you can sell the house. 

We would like to throw a word of caution here that as mentioned by you, your sister is not signing any NOC / papers and tells you to sell the house, it clearly indicates her ill intent to create trouble for you and your other brother and sisters at a later point of time. While the sale of the joint property cannot be done legally without her consent, even if you manage a way out to sell the property without her consent, it is liable to be challenged by her at any time later and will create trouble for you and / or the purchaser of the said property.