Legal Question in Wills and Trusts in California

Genetics do not matter in the law other than to establish or disprove disputed paternity. However it appears X may have some claim on the estate. Your mother's parents are listed as the parents of X on his birth certificate because they adopted him, then he legally is your and your deceased brother's uncle. If he was not legally adopted and therefore the birth records do not accurately reflect legal parentage, then he probably is your and your deceased brother's legal sibling. Without a full examination of the family tree and dates of death, it is impossible to say for sure if he has a claim that would be successful, but he certainly has standing to assert a claim to a share of the estate since it appears he is either legally a brother or uncle of the deceased.