To the extent that apportionment of an estate tax is not controlled by an instrument described in RCW 83.110A.020 and except as otherwise provided in RCW 83.110A.050 and 83.110A.060, the following rules apply:
(1) Subject to subsections (2), (3), and (4) of this section, the estate tax is apportioned ratably to each person that has an interest in the apportionable estate.(2) A generation-skipping transfer tax incurred on a direct skip taking effect at death is charged to the person to which the interest in property is transferred.(3) If property is included in the decedent's gross estate because of section 2044 of the Internal Revenue Code or any similar estate tax provision, the difference between the total estate tax for which the decedent's estate is liable and the amount of estate tax for which the decedent's estate would have been liable if the property had not been included in the decedent's gross estate is apportioned ratably among the holders of interests in the property. The balance of the tax, if any, is apportioned ratably to each other person having an interest in the apportionable estate.(4) Except as otherwise provided in RCW 83.110A.020(2)(d) and except as to property to which RCW 83.110A.060 applies, an estate tax apportioned to persons holding interests in property subject to a time-limited interest must be apportioned, without further apportionment, to the principal of that property.(5) If the court finds that it is inequitable to apportion interest and penalties in the manner provided in this chapter because of special circumstances, it may direct apportionment thereon in the manner it finds equitable.