Is it permissible to speak about her husband's past with his sister (who was his mistress) and consider that a scandal?

As long as the husband has repented sincerely, the wife should have a good opinion of him, live with him, and cooperate with him in obeying Allah. She must conceal that woman and not expose her, as that is forbidden. As Ibn Allan said: "As for her exposing her to anyone other than a judge, such as speaking about her, that is backbiting of severe sinfulness and burden."