thre is no such thing in law as next of kin - the only reference is in the mental health act which defines the "nearest relative" thus:
husband, wife or civil partner (including a person with whom the patient has been living as for not less than six months);
son or daughter;
father or mother;
brother or sister;
grandparent;
grandchild;
uncle or aunt;
nephew or niece
in that order.
In reality, in the event of the parent dying, a child might not necessarily be cared for by the "nearest relative", or a relative at all. I would imagine social services would become involved and a determination made in the best interest of the child, so for example if te step dad was willing to carry on looking after her, ss/family courts may determine it was in her best interest not to split her up from her sister If you had an objection to this, then i woud imagine it would go to court. So "next of kin" is a red herring really