Property Issue

My father have 1acre land and he is an alcoholic. He have some friends who is also an alcoholic and taking him to the wrong direction. I have also admitted my father in rehabilitation center twice but of no use. Now my dad says he will give the property to someone as per his will. I have build a house in my fathers property (house construction not completed). However, the property bought by my dad after his marriage and my mom says dad have sold her gold and took her some money to buy the land. So I would like to know is there any possible ways to stop this happening? Like giving to others without our knowledge? We are 4 family members(Dad, mom, younger brother and myself) Original property document are there in a bank locker of mine because we are afraid that dad will give it to pawn brokers (Dad have no job) I am also afraid that after my dads demise, people may come ask for money/dispute property that he borrowed so and so. Because normally he have the habit of borrowing money for his drinks.