Custody

Hi all, My sister got married 1998. Due to dowry harassment she return my home 2002. Myself gave all facilities and studies to her and her son. 2008 her husband filed divorce but he black mailed so she not ready to live with him. So divorce petition ex partebut she studied well got two degree and started work in school. Suddenly her ex husband came 2010 and promised her forgive all matters and to get remarriage. She believed him and went to him with boy child. But not remarriage She got pregnant and second girl baby 2012 but her in laws ill treatment she sent her boy child to my home and requested take care of him. Till now he with us. 2016 my sister suddenly sick after several treatment she died may 2017. Before death she requested my mother to take care of her son and daughter. I have some question here: 1. She nominated all her PF and insurance to her son name. 2002 to till date he living with me. My mother only spending all study expense to him. And my sister never get remarriage to him shall get her baby girl from him because her last request. My mother get guardianship for both daughter and son. Shall we apply maintaince for the children. Please advice.