(a) Shares of beneficiaries abate in the following order:
(1) Property not disposed of by the instrument.
(2) Residuary gifts.
(3) General gifts to persons other than the transferor’s relatives.
(4) General gifts to the transferor’s relatives.
(5) Specific gifts to persons other than the transferor’s relatives.
(6) Specific gifts to the transferor’s relatives.
(b) For purposes of this section, a “relative” of the transferor is a person to whom property would pass from the transferor under Section 6401 or 6402 (intestate succession) if the transferor died intestate and there were no other person having priority.