Child Custody for the father

Hi, I was divorces last mid year. I paid Alimony and agreed to pay child maintenance. I have 2 daughters and I have been taking care if their financial expenses from day one. Even when I was married she never spent a penny. My Ex wife already was in a relationship when the divorce was going on and I never knew of it. I agreed to the child custody trusting her words that she would never get married. Within 2 months post the divorce I got to know ahe is in a relationship. She is married now and I have asked her to transfer the custody of my girl child. She has refused. She has been taking my kids to her then boyfriend house even without taking my permission. When I confronted her she filed a police case saying I was interfering in her life. My kids come home every Friday night and leave to school mondays. My younger one comes back to my place every day post her class by 11.45. I do all things from lunch to bath to studies and drop her in my ex inlaws place before leaving to work by 5pm. Kids love me a lot as I have been handling all their responsibility since they were babies. My question. 1.Why can't I get the custody as iam teh biological father. 2. I will stop the maintaince for the kids since my ex is planning to move them out of their current school for her convenience to the other side of Banglore against my will. Iam willing to take all the responsibility for my kids post the transfer of Custody. Note kids are 6 and 11