I would say it depends on the wording. If it says I leave my estate equally to my nieces A B C D & E and another niece F is born later, she gets nothing. If the will says I leave it to all my nieces then 1/6th each.
In the first case If A to E all give up their share then it still cannot go to F as she is not a beneficiary. But I have no idea who it would go to, the person who disclaimed last? If A to D disclaim on a Monday then it all falls to E, she cannot disclaim a 100% share?
But what has actually happened so far, what does the will say and who has disclaimed so far?