Open Title House

The house was in my mother in laws name. Mother in law expired 3 years back. Without any will. My father in law also expired a few days back. My husband has 2 sisters who have married. Now after my father in laws death. One of my sister in law came with her husband and daughter and started living in the house in my father in laws room. Can she do it? Without any court order? Is this not trespassing? I am feeling awkward because we are staying here and were looking after my inlaws till their old age death. Now my sister-in-law is not alone divorce or widow but with a husband who is totally an outsider for me. I am not feeling any privacy. What should I do plz suggest?