divorce and blackmailing

My name is jahangir my sister want to take divorce n custody of her childrens who r minor..initially my sister husband has an martial affair with another girl(my cousin sister) on that we planned to file a case but he blacmaild my family.. After that he started giving torcher to my sister.. Becoz of that my sister started a affair with some one he came to know the same now he done with his second marriage..n now her husband is not agreeing to give the custody of children n he is done with his second marriage without any legal divorce formalities from her first wife (my sister).. We want to claim the custody of children n demand an amount of money for better growth of children we wnt to file a case against him but he is blackmailing my family so plz help me how i need to proceed..

Thanks