Sister being tortured and blackmailed by inlaws

Hello please suggest me some points .My sister is married 6 years ago with 2 kids one is 4 year and one is 3 month.she got married by deception as husband lied about education and age.my brotherinlaw is very shrewd person and has black money currency business .My sister got upset on all this and came to my home and then my brotherinlaw pleased her somehow and they both went back to live in inlaws home. Later They started asking money from my sister which my father fullfilled by giving them activa or gold when their kids were born. Recently my dad passed away and my brotherinlaw has threatened my mother that my sister has a boyfriend (their driver)and the 3 month child was her boyfriend.He is black mailing and beating my sister as he got some naked pictures of my sister in her boyfriends phone which he has snatched from her boyfriend He asked 2 lakh from my mom for Dna test He showed few pictures of her to my mother and my mother got afraid and gave cash 1 lakh to him and then their plan changed . My sister and brotherinlaw never had any Dna instead they used money for something else . now they both are demanding next 1 lakh from my mother and brotherinlaw has threatened my mother for murder if she dont give him cash which we have recorded on phone.Now everyday my sister is crying on whatsapp in front of mother that she is being tortured to bring cash from my mother but never takes any step against her husband. When my mother seeked help from police about cash demand my sister changed her statement that she is happy with her husband and he hasnt tortured her. Maybe she is under his pressure not to open her mouth otherwise her picture will be leaked.My sister phone’s sim card has been taken away by my brotherinlaw and she is communicating with my mother only on whatsapp.Please suggest what can my mother do to take my sister out of that situation as my sister has no strength to voice out. thanks brother