ancestral property illegal sale

if you are sure thy have sold through illegal document then file a suit for cancellation of sale deeds with respect to that property and go for partition suit now it or file a criminal case against cousins who have done this.

First get all the documentary evidence, then issue a legal notice to your father's cousin. Since it is an ancestral property, you have a share.

If they are not authorised to sell, you may lodge a complaint with the Special cell for anti-land grabbing in your district headquarters or if they are also a share holder, you may file a partition suit demanding your legitimate share out of the ancestral property, if the property is confirmed to be sold then a suit for  declaring the sale as null and void will be applicable.