Property issue

Dear experts, My grandmother is around 70 years and she has a property worth 20 lakhs which was rented to somebody else and she lives with that rental money.she had signed papers to elder heir that she will be giving that property to him after her death without realizing that he just needs her money .after few years she realized he is not taking care of her and when she was in a postion to die she registered the property to my father to take after her death.now the rented property was vaccated forcibly by the elder heir and has sent lawer notice saying he is collecting rents since long time and showing as that property belongs to him which is not true.his goal is to run this case in court which will take 20 years and take that money by the time she passed away. When she is trying to get to her property area he is pushing her away and even filed case at police station but of no use.police gave the lock of property to him and to my grandma.we dont know what can be done to vaccate that place . Advice will be appreciated.