For the purposes of this article:

“Gross estate” includes any property or interest that is required to be included in the gross estate of the decedent under the estate tax law of the United States, increased by any “adjusted taxable gifts” as defined in § 2001(b) of the Internal Revenue Code.

“Persons interested in the estate” includes all persons, firms, and corporations who may be entitled to receive or who have received any property or interest that is required to be included in the gross estate of the decedent or any benefit whatsoever with respect to any such property or interest, whether under a will, by intestacy, or by reason of any transfer, trust, estate, interest, right, power, or relinquishment of power taxable under any estate tax law of the Commonwealth, any other state, or the United States heretofore or hereafter enacted.