He could have been saying a lot of things.
He could have been saying that Thompson has been active with women after his first marriage. After all, he used the present tense. He could have been saying that there was infidelity, but that it was not the cause of the marital break-up, as you implied. He could have been saying that Fred Thompson was seen in the company of other women, and this was a cause of marital friction, without implying he was unfaithful with those women.
What he pointedly did not say to the question of whether or not Fred Thompson was unfaithful was "yes". In fact, what you have here is a man who seems to be very careful to not say "yes". And yet, you take that as a definitive "yes".
That, to me, is a pretty thin thread to be making an accusation, as you have.
Now, I am not saying that Fred Thompson was faithful in his first marriage. I have no way of knowing whether or not that is true. But to leave that question behind, and blithely move on to the debate over whether or not his supposed infidelity in his first marriage is relevant to the current political campaign seems to me to be making a pretty big leap.
Fred’s first wife’s family are rock-ribbed, influential Republicans. They got Fred his political appointments, and helped him in his political career. They probably like his politics, as does his first wife. She wouldn’t be the first wronged wife to keep silent for political reasons.
I think the brother’s answer was pretty clear, viewed in context. I could be wrong, I admit, but I think the chances are small.
I’m really surprised, though, that so many people who consider themselves religious think that promiscuity is okay, though, just because a man is single.