Section 2-1.3. Adopted children and posthumous children as members of a class

(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.