Seller's son is trying to grab our farm land

Sir We have purchase a farm near Indore MP approx 2 year back and got it registered. Also we have got the property mutated in our name ( we have the documents with us but when we check online it still bears the seller's name). Seller was around 70years old at the time of registry and was fit. Now 6months ago he passed away. All the payment was given through cheque except a small amount. Now his son who has his land near by which he received in decendecy, very often starts to sow seeds in our part of land. He says " I gave possession of some part of my land as the land you purchased from father was less on the spot" . Also he gave my mobile number to his sister who is married somewhere far and she started calling me and says I want my part from my fathers property and did you purchased. I feel all the family is trying to grab some money from me. I tried to put fence around the land few days back but they sent some boys to threaten the workers and managed to stop the fenceing work. Plz suggest what should I do.