regarding divorce and blackmail

My name is jahangir my sister want to take divorce n custody of her childrens who r minor.. But her husband is not agreeing to give the custody of children n he 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 how i need to proceed..