Either:
(Select text to see answers.)He is an adopted child who has legally been made into a family member of the child that was not born.
or
He is an adopted child who was legally made into a family member of the child that has not been born.
Both are grammatically possible, although the first one would make sense only if you were talking about a wealthy aristocrat who left all his money to his unborn son, but the pregnancy miscarried or was aborted, and an adopted child became the heir, or something equally complicated!
The second sentence would describe the more "normal" case of a husband and wife who adopt a boy while the wife is pregnant; the boy becomes the legal brother of the unborn baby.