Hi Sameer,
I want to say only one thing to You finally.We can not produce sound with single hand ok. First analyze the matter from both sides. In any disputes both parties mistakes will be there.you (Your sister) have following below options.
1. If your brother-out-law really showing crueality then take mutual divorce, Such type of guys will never change and dont expect chnage from them. It will be increased if ur sister went to his house in the present situvation.
2. If any mistakes are your sister side try to make understand her mistakes, And told to her speak softly to ur brother-out-law,and ask forgiveness her out-law's also.If ur's brother-out-law's parents forgive her the problem will be solved easily.
3. If you people promised anything in the marriage please settle those things first.Most of the marital issues come for this reason only.
4.From your side, First learn how to respect ur brother-out-law(brother-in-law).And try to speck patiently and smoothly to ur brother-out-law,if he is angry also.Mostly dont involve in between ur sister and brother-out-law. People like u involve in these matter's and make worser the situvations.
5. Most importent thing ,Don't put false alligations with lawyer's or police advices. This will crack's all the relations.Your brother-out-law maintain distance from you people.He get irritated to see ur sister's face if you people keep false alligations.
6. Get a clarity from ur sister,What she needed ,she want to save her marriage or she want to take revange.If she want to take revange then it is like throwing stones from glass building . If she want to save her marriage need to decrease her ego, otherwise she is the looser.You and your parent's need to maintain her and her kids for life time
7. Most importent thing, Law is not blind. Judge will not hear your cock and bush stories ,He will give next hearing date simply.You will give good luxury life to one lawyer. Finally your sister will get divorce without alimony(If she put false alligations).If she kept right alligation and those are prove your brother-out-law will went to behind bars In this case also you and your family need to maintain her for life time. Your sister's kids life will be spoiled. For these thing will take nearly 5 to 7 years.
Then you can decide yourself ,What to do ,What not to do.