The first step would be establishing how title to the interest was held at the time of your natural father's death. That will determine whether or not it was a part of his probate estate and, thus, whether or not part of the interest would have become yours upon his death, subject to the probate of his estate. You can't inherit what he didn't own upon his death. If he transferred the interest to his 2nd wife during his lifetime (by placing it in joint tenancy with her and granting her rights of survivorship, for example), it was not part of his estate upon his death and, thus, not something that would pass to his heirs by virtue of his death. If it was titled in his name at the time of his death, then the next step is to determine if his estate was probated and, if so, what went on in that proceeding.