Character mistake: Sidney asks on the porch why Cotton couldn't prove that he was having an affair with her mom. Tatum's answer is "Well, you can't prove a rumor. That's why it's a rumor." That's quite the preposterous answer, especially since the point she wants to make is that it was not in fact 'just' a rumor.
Suggested correction: Tatum isn't trying to argue with Sidney. Tatum agrees that she doesn't know the truth about her mother's infidelity and certainly can't prove anything. Rather, what Tatum is trying to tell her - gently and somewhat indirectly - is that she believes the rumors, which is distinct from knowing the facts.