Is there any sin upon one who, out of ignorance, referred to his fiancée as "my sister," and is expiation (kaffarah) required of him for that?

There is no harm in the fiancée calling the groom's mother "my mother," nor in the fiancé calling his fiancée "my sister" before the marriage contract. Although it is disliked for a man to call his wife by the name of someone forbidden to him (such as his mother or sister), it does not make the wife unlawful nor does it establish the ruling of ẓihār. It should be noted that the fiancée is a stranger to her fiancé before the marriage contract, and moderation in love and attachment is required.