Chemish is Amaron’s brother, not his son. It is probable that Chemish had no son, or at least no living son, who could receive the plates. The plates stay in the lineage, but take a half turn to a brother rather than a son. This transmission of plates, or even of authority, will be seen at various times throughout the Book of Mormon. The preferred transmission was father to son, but in the absence of a son, or perhaps for some other unknown reason, the secondary option was to go through the brother’s line.
Chemish has even less to say than his brother or father. That may be the result of receiving the plates later in life. He notes that he saw what his brother wrote. After noting that the transmission is intended to remain in the family, he felt he had accomplished his purpose. Perhaps he was not much younger than his brother and did not have long to keep the records. He doesn’t tell us. He ends after this brief statement: “And I make an end”.