Illigal construction in agriculture land

What were you doing for the last 20 years if the subject property was acquired by your grandfather ?

Contact local advocate for due guidance.  In Telangana, the situation is entirely different when compared with other states.  (then PM PVR is  also not an exemption)The landlords fled from villages to Hyderabad due to Naxal occupation, and their grandfathers expired, and none of the legal heirs had never a chance of visiting the agricultural lands.  The issue was there from 1969.  If the legal heirs got knowledge of the site occupation only recently, there may be a chance for fighting.  The success depends on facts, circumstances, and the sincerity of Advocate in presenting circumstances facts in his plaint. 

In one case one of such cases, a  palace (Devdi)building was entrusted to his brother in law, the brother in law was collecting rent and sending it to the landlord at Hyderabad(Legal heirs of the landlord never visited the village as a suspected Naxalite was crushed under a tractor, and the blood-stained tractor was confiscated, the family fled from the village, and even after 40 years the relatives of the victim are trying to take revenge on landlord's family).  The rent was Rs.2,600/- p.a.  When recently his grandson went to see their roots, he has learned that it was in some other's possession since 1971!!!!!

Theoretcally, you can approach the court to seek their eviction. It is for the other side to plead and prove their rights or claims, if any. You alone culd be the judge whether such proceedings would be prudent or otherwise.