Need guidance

My brother in law killed my sister and managed my father ( alcoholic) to not to file case ( below 7 years dowry harrassment case also not filed) . At that time no police came to me and my father lied to me that he filed case. I came to know the truth after 3 days. Then I have visited police station multiple times later compromised with my destiny and taking care of these children. But no use. Now that killer wants his children after 1 year. I have doubt that he will kill them also. But as per law father is natural guardian. How can I Fight. Children aged 3 years and 5 years. They are staying with me since 1 and half years. They also don't want to go to their father. Please guide me. Another thing sir my brother in law proposed me for second marriage when my sister is alive. Please guide me to save these children life sir. Another question sir I can go with private complaint but jurisdiction completely belongs to Accused. Accused frequently stacking near to my office to do something harm to me and kids. I am afraid to go to that jurisdiction. If I directly go with high court then there are chances of being rejected because I have not approached lower court. I