Selling grandfathers property by our father without our signature

Dear Sir, Kindly advice on our critical situation as explained below: Brief About the Property: 1. This is our grandfather property which is currently under control of our father (Please note, all the documents are in our father’s name) 2. Three months before our father had registered an agreement with one person for selling the same property 3. The above said agreement consisting the following points: a. The buyer had paid the 50% of property value at the time of agreement b. To own the property, the buyer should pay the balance amount (Balance 50%) within 5 Months from the date of agreement. c. The above said agreement been signed by my father d. My mother had signed as a witness e. Other three third parties had signed as a witness (Two for us + Two for Buyer) f. The above said agreement been legally registered Current situation: 1. Earlier our father had decided to sell this property due to the problem our loans. 2. Currently after lot of struggles, I had settled our family problem by getting some help from my father in law. 3. Actually we are three brothers (My self - Major + 1 Major + 1 Minor), the above said property is very important for us to survive. 4. Now, we had convinced our father to not sell the property and settle the money which we had taken from buyer. The Problem: 1. But the buyer is not ready to take the money & he is willing to buy our land. 2. We had requested many times but he is not accepting 3. Now, he is saying he will pay the balance money (Balance 50%) in court and he will enter to our land. Now my questing is: 1. Actually I didn’t sign & my brothers also did not sign in that agreement 2. Also the agreement was only signed by my father 3. Since, this was from our grandfather Kindly advise: a. Does our father has a right to sell this property without our signatures? b. Is it legal, if the buyer settle the money in court? c.Can he enter to our land without our signatures d.Can I file a case if he enters after settling the amount. In view of above, Please advise me how to proceed and kindly let me know about my stand on the same.