Proper house in Kolkata city. Dispute over ownership

My father and mother both had a 4.5 cottah land with house in their name. We are two sisters with no brother. Now, after my marriage, I shifted to my husband's house with my newborn son, but then my sister and her husband shifted here and started living in our along with my mother and father in the house despite my mother's obstruction. I thought that she was my sister and it's alright. But, suddenly on 2017 my mother died intestate. And, as usual they lived there with my paralyzed 80+ father who can't walk. Being a govt retiree dad recieved good monthly pension and they started handling all bank accs. I seldom visited there. But, then suddenly I came to know that sister have registered dads part in her name. Now when I go there, they have put a lock in the gate and do not let me in. What can I do to stay there please help me.