Sid is Tom Sawyer's whiny half-brother, who also lives with Aunt Polly. He behaves well, but enjoys getting Tom into trouble. He appears to be around nine or 10 years old, and lies.
In Adventures of Huckleberry Finn, Tom impersonates Sid in order to explain why two boys and not just one had visited Tom's relatives. Huckleberry Finn was the other boy, and he had already lied and said he was Tom.