Sibling custody rights/obligations
I have a query about my rights and obligations regarding custody of my younger sister. I am a 19 year old female, and I have a younger sister who is currently 15 years of age. My father is dead, and my mother is suffering from a terminal illness. I know this probably sounds really strange, but I whether I would have the right to custody of my sister in the event of my mother's death? Or would this be expected of me? My mother hasn't any Will and I suppose I am next of kin and a ''major''. I'm studying nursing at the moment, have no criminal convictions, no arrests, no warnings, I'm not married but I have a boyfriend. I am really confused about this situation, and I don't know what would be expected of me or what my rights are. There are other relatives (Uncles, Aunts, Grandparents) on my Mother's and Father's sides of the family, but I am worried about my sister's happiness and whether she's really want to live with them.
I really hope you can help me, because I'm really worried about this!