Biological Siblings

She has one full sibling and five half siblings.
Yet she is an only child.
Some of her siblings live with their father, some with grandparents and some with their other parent.
None live with their biological mother.
She complains to me frequently that she wants a brother or a sister.
She has them but does not know about them...yet.
How do you explain to an almost seven year old child the dynamics of her siblings? Why do some live with her biological father? More importantly why doesnt she live with him?
I know I am going to have to explain it to her in the future, when she is much older.