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I am sorry for your loss and for this difficult situation.
Your niece is not the executor as there is no Will and if anyone applied to the court to administer the estate it would be you as only a spouse or child would inherit and not a niece.
But it sounds like she may want to keep the money that is not hers to keep and you need it to pay the funeral expenses which come first, even ahead of taxes.
So it may make sense for you to pay a lawyer to send her a letter demanding that she forward all funds she has that are your father's as well as a full accounting for the funds that she took care of and, if necessary the lawyer can threaten a lawsuit.