My sister is experincing mental violence, Please help

Dear Sir, my sister marriage completed approximately 2 year. Boy's Parent's does not tell us that he is diabetic 1 pateint . My sister come to know after marriage but we have no evidence of this how old this disease to my Jija Ji. In addition to this , My Jija ji asked my sister that he does not like us indirectly he want to say to my sister to break relation with us. Whenever my sister call us , he is always with my sister , Jija Ji does not give my sister freedom.My sister asked she wants to break relation with him , but he threatens that he (Jija Ji) will suicide. What can she do legally?