Is the sister considered divorced after she cursed her husband, who had told her: "If you curse me, you will no longer be under my marital bond," knowing that he forgot what he said and remembers nothing?

It is not permissible for a wife to insult her husband, as this entails harming a Muslim and incurring sin. A believer holds greater sanctity with Allah than the Kaaba. The phrase "you will not remain under my responsibility" is an implied divorce, which does not take effect except with intention. If the husband does not recall uttering it, then there is nothing against him, because the default is the absence of intention. The questioner's sister is not divorced now. She should have informed her husband about the conditional statement to ascertain whether a divorce occurred or not. There is no sin upon her due to her ignorance of the ruling.