Procedure to take child custody after filing for divorce

My sister has been married for 7yrs. She is a gynecologist by profession. She has 2 daughters and always had a mentally abusive marriage. Her inlaws and husband are very money minded. They want her to earn and give them all the money and then they would not let her spend a penny on herself and her children. She does not want to get dragged into a legal battle. Her husband would be more than fine to divorce her, but they might create issues in child custody. I want to know under which circumstances the court can give the child's custody to the father. Her daughters are of 5.5yrs and 1month respectively. We at no cost want visiting rights with father. My sister does not want any alimony. Do we need to file a case of mental abusive marriage or simply a divorce application can do and she can get both children's custody?