My mother in law is raising her granddaughter as her own?

my niece thinks that her real mother is actually her sister and her suppose to be grandmother is her mother, we need some advice on how to tell a 6 year old that her "sister" is actually her mother!!!!!! the real mother does not want no part in raising the girl as hers she already has 2 other children, that by the way they are telling her is her brother and sister. they are all thinking that they will never have to tell her the truth. i finally got my mother in law convinced that they should break the news while she's young, so maybe it won't be as hard. How do you explains a situation like this?? please help.