Child custody- Sister vs Sister

[AnitaHess]

I would like to file for custody of my 12 year old niece who has lived with me since 7 years old. After divorce mother and father made a reckless decision to send her to my other sister who is a stranger to this child. My sister now is trying to sever the bond b/me and the child by not allowing contact. Mother couldn't take care of the child and the father wouldn't. This is now sister vs sister and the welfare or best interest of the child was not considered. Can you tell me how to file this and of any cases involving sister vs sister in custody situation? State is Nevada