Bava Batra 134 - Identifying Someone as Son or Brother
If one says, "This is my son," he is believed. What is the practical consequence of this ruling? If it is to make him inherit, then this is obvious since the father could have given him a present. Rather, it is to make the man's wife free from the obligation of levirate marriage to his brothers.
If one says, "This is my brother" - the other brothers don't have to believe him and share their inheritance with the newcomer. The one who recognized the brother, however, is obligated to share in the inheritance. If the newly known brother dies, his properties revert to the one who identified him, but not shared with other brothers. However, if the new brother inherits or acquired new properties, the other brothers inherit them. How could it be? Because they said, "We don't know."
Art: Harold Copping - Sam Weller and his Father, 1924