Illegal possession of holding from last 6 years

Hi Madam/Sir, We are facing an issue that one lady illegally processioning a part of our house and she is claiming that she is wife of my younger brother with some weeding photos, but no marriage registration. When my mother in my house Delhi some of my brother friends purposefully help the lady to enter in my mother's house by breaking door lock in Darjeeling 6 years back. After few days she beats my brother and asked to leave the house else she possibly kills him. He left the house and still now she captured a part of the house and not ready to leave the place after three notices. Now she collecting other rent too. My mother age is 65+, father expired in 2009, House was is in Mother name and father name. We are two bother and one sister. Mother's source of income is the rental income from that house. So she is really in pressure how to recover building from the Lady and what will be the steps, how many days it will take if she wants to go legally. Kindly suggest. It would be really help to my mother and us a lot.