Proposed marriage of brother

Sir, I will give the scenario in short. My mother is currently in a state of mental imbalance-diagnosed with schizophrenia. My mother is staying in her own house along with my brother . she is unwilling to leave her property and come and stay with me as she is obsessed with the property. Currently her food and clothing are handled by me with the help of third party food services.My brother ,whose relationship with me is not good , is currently entering into a matrimonial relationship. its a love marriage.Hardly a year since he met the girl. I had already told my brother that marriage was his responsibility and i cant take any burden of the same. Shares pertaining to property will be rightfully his after the demise of my mother and not before that. As long as she is alive its her right to stay in her own house with peace.The girl concerned is of a typical character. From day one onwards she started isolating my brother by making him quarrel with his friends, with my mother and eventually with me. She had forcefully come to meet me and my wife and complained to us that my brother was mistreating her by using abusive language. Subsequently my wife called up the girl's mother and informed them to reconsider this proposal under the light of these developments. My brother had told lots of lies about his educational qualification,wealth etc. My wife gave point to point clarity on these things. The girl and her mother recorded the phone calls and showed it to my brother. It led to escalation of tension between us. In the meanwhile my brother started using abusive language towards my mother and also started hitting her. The girl also started threatening mother by coming to the house and also over the phone. My mother is having great motherly affection towards my brother and due to this she is not complaining to the authorities and have asked me also ,not to do the same. This girl and my brother, they want me and my mother to conduct their marriage in the presence of all relatives, which we are not interested in doing. As i am aware about the negative character of the girl,i am apprehensive about the safety of my mother.This girl is constantly engaged in recording conversations with all my relatives(as per threats given by my brother) . Now my brother has started blackmailing me by saying that this girl will file a case of harassment against my wife ,using the earlier call recordings as proof. If this shouldn't happen,then i will have to conduct their marriage in the presence of my mother and relatives Sir, i dont know what their intentions are. I have already given permission to my brother for a registered marriage.Already told him that he will get his share of property after the demise of my mother. But still he is threatening me and my mother. This girl is recording all telephone calls and creating rift between me and my brother and threatening my mother also. And now he has started blackmailing me. Why are they doing this ? Could there be any ulterior motive?I have a feeling this is some how related to the small property we are having,which is being targeted by the girl. Can this be tackled using any legal resolution?