The countess was deluded rather than sinful.

Eugenie feared to be treacherous and base in revealing secrets that society (agreeing on this point) holds to be inviolable; but--she saw her sister's future, she trembled lest she should some day be deserted, ruined by Nathan, poor, suffering, disgraced, wretched, and she hesitated no longer; she sent in her name and asked to see the count.
Felix, astonished at the visit, had a long conversation with his sister-in-law, in which he seemed so calm, so completely master of himself, that she feared he might have taken some terrible resolution.
"Do not be uneasy," he said, seeing her anxiety.

"I will act in a manner which shall make your sister bless you.

However much you may dislike to keep the fact that you have spoken to me from her knowledge, I must entreat you to do so.

I need a few days to search into mysteries which you don't perceive; and, above all, I must act cautiously.