Get divorce and children custody as well as all compensation

Hello Sir, My sister got marriage in 2010 in UP basti, she have 1 son as well as 1 daughter and we are stay in mumbai from past 20-25 years. After 2 year once my sister came mumbai that time she was pregnant after some days she born baby girl, meanwhile we heard that may sister husband get marriage with other girl without informing us. We are try to contact them but they are not ready to talk with us, then my father went UP and contact them then they clearly told us that they neither give divorce nor any expenses whatever you can do do that we wont give and also we will take my son after some days, they only want son not daughter. Then we lodge case against them in mumbai court but nothing we get its around 2-3 year but nothing happen our lawyer says they didn't take any court notice, without taking notice we cant do anything. They suggest us lodge complaint against them from UP then only something can happen. Since we don't have any contacts in UP it is very difficult to us to do this kind of things from UP. So please suggest us what can we do, and if I lodge complaint from UP then which IPC is needed for immediate action because we cant stay in UP long time as me and my father is working in mumbai I cant get more then 1 moth leave from our company. It would be grate if you suggest me something that can help me to out of this hell. Thanks in Advance,