• In-laws and husband harassing and threatening to file a divorce

Our family lives at Bokaro, Jharkhand and My sister married in 2014 at Kolkatta, her husband works at Airport authority as a clerk and initially everything was ok with her except few minor disputes between her and her husband. 
lately i come to know from our mother that things are not going well in my sister life, she didn’t tell anybody in our family about it because our father has expired last year and she didn’t want others to be worried about her.  She told us that she feels that her husband is having an affair with someone else and his behavior along with her in laws have changed a lot. He used to taunt her always whenever he was drunk that he has married to some other girl and soon she will be of no use. Thereafter they started abusing her and in-laws also started harassing her mentally and physically .  we used to consol her as to save her relationship.
all of a sudden 4 months ago her in-laws have left her without any reason to our house at bokaro saying that they would come next day and take her with themselves. but thereafter they didn't came nor any of them picked call of any of us. We all were very worried and so after 1 month my brother and sister went to their home at kolkatta and found their home locked, they checked with the neibhours and then come to know that nobody her in-laws were living in that home since the day they took my sister , however some of neighbors had seen their families 3-4 times at home that too in night with a new female and new-born kid (in-laws family consist of 1 elder son, husband of my sister,  younger Son 11 years of age and mother and father), my sister has few photo of the girl which she got from her husband mobile and she shown her photo to their neigbour and they confirm that few of them have seen their family with the girl and child. Then come the story that the guy is having affair with that girl since long and everybody in his locality is aware of this, only my sister doesn’t know because she wasn’t have the permission to go outside or to talk with anyone. Neighbor also told us that the girl has filed a divorce against her husband few years back and currently struggling for separation, her husband is some labour / worker lives in kolkatta . 

 So my brother and sister returned back after waiting two days, we thought of filing the case against them but since we were financially weak after my father demise and also we want to somehow save our sister marriage, we waited for speak with them. Then one day his father picked my call and said that they will not take my sister to his home, if I want to do case or anything, I’m free to do as they don’t fear for anything they can fight case as long as it goes, they are financially strong. 

Again as to somehow make the things working, my brother and my sister went again to their Kolkatta house but this time only my sister inlaws were there and there was no sign of her husband. And they didn’t open the gate and didn’t let my sister inside, and to our surprise- they started abusing my brother & sister from the roof of the house and said that they have filed a diary and soon they will file a divorce case and they called local cops also when things got into heated arguments. And local policeman told us that they have file a diary of section 107 & 144, so we must leave their house immediately or they will arrest them, better to go to the Court. We were totally shocked; my sister inlawas have taken whole jewellary from my sister saying that she’ll deposit in locker,they have taken a huge sum of cash as dowry from my father and to arrange that my father have to sell land property and now even after their demand they are treating us like this, we all are very sad about this whole incidence and don’t know how to proceed with the case. 

I request some of the experts to please come forward and guide us what we should do in this case.