Custody

My sister got married 1998. 1st child boy born 2000. After 8 months my sister and sister husband requested my parents to take care of 1st child. So, my parents and myself taking care off child. 2008 my sister got divorced from her husband. Afterthat she also not take care of boy baby. Also father of boy baby. 2010 my sister got job her ex husband place. They started living to gether. 2 girl child born out of marriage. 2012. My sister passed away 2017 due to cancer. Afterthat father of both child. Take care off only girl baby. Boy living with us and we spending money for his education last 20 years. Meanwhile father of children not allowing to see the girl baby age of 9 years. He not taken of her properly. Education, food etc. Shall we get legal custody for the girl baby? Some rape incident happened near his home.. we bothered her safety..