(a) Unless the creator expresses a contrary intention, a disposition of property to persons described in any instrument as the issue, children, descendants, heirs, heirs at law, next of kin, distributees (or by any term of like import) of the creator or of another, includes:

(1) Adopted children and their issue in their adoptive relationship. The rights of adopted children and their issue to receive a disposition under wills and lifetime instruments as a member of such class of persons based upon their birth relationship shall be governed by the provisions of subdivision two of section one hundred seventeen of the domestic relations law.

(2) Children conceived before, but born alive after such disposition becomes effective.

(3) Nonmarital children. For the purposes of this paragraph, a nonmarital child is the child of a mother and is the child of a father if the child is entitled to inherit from such father under section 4-1.2 of this chapter. The provisions of this paragraph shall apply to the wills of persons dying on and after September first, nineteen hundred ninety-one, to lifetime instruments theretofore executed which on said date are subject to the grantor's power to revoke or amend, and to all lifetime instruments executed on or after such date.