(They said, “O father! Indeed, we went racing with each other, and left Joseph with our possessions. Then, a wolf ate him. And you would never believe us, even if we were truthful.)
The brothers of Joseph told their father, Jacob, that they went to race with each other, and left him with their possessions. Then a wolf ate him when they were absent. They said that their father would never believe them even if they were truthful in their statement because of his great love to Joseph.
It was also said that you would not believe us even if we were truthful because you did not see a sign indicating our truthfulness.
(…even if we were truthful.), this sentence reveals that they were lying. If they were truthful, they would not say that as truthfulness testifies to its reality.