For purposes of the tax imposed by section 2001, the value of the taxable estate shall be determined by deducting from the value of the gross estate the amount of any estate, inheritance, legacy, or succession taxes actually paid to any State or the District of Columbia, in respect of any property included in the gross estate (not including any such taxes paid with respect to the estate of a person other than the decedent).
26 U.S. Code § 2058. State death taxes
(a) Allowance of deduction
(b) Period of limitationsThe deduction allowed by this section shall include only such taxes as were actually paid and deduction therefor claimed before the later of—
(C) a claim for refund or credit of an overpayment of tax imposed by this chapter has been filed within the time prescribed in section 6511, the latest of the expiration of—
60 days after a decision by any court of competent jurisdiction becomes final with respect to a timely suit instituted upon such claim, or
2 years after a notice of the waiver of disallowance is filed under section 6532(a)(3).