An unadopted stepchild may establish that the decedent was his parent for purposes of distribution of his intestate estate by proving the requirements set forth in either the Uniform Parentage Act (Fam. Code 7600 et seq.) which is incorporated by reference by Prob. Code 6453 or the separate requirements of Prob. Code 6454.  So the lack of a legal barrier to adoption–a requirement under Prob. Code 6454–doesn’t block proof of parentage under the UPA.