Does expiation become obligatory upon a woman who swore to her husband in a moment of anger that she would not attend his sister's wedding, and what is this expiation?

If you swore not to attend your husband's sister's wedding and then attended, you must expiate your oath in the following order: feeding ten poor persons, or clothing them, or freeing a slave. If you cannot find any of these, then fasting for three days. However, if the oath was about the future and you have not broken it, then nothing is incumbent upon you if you do not attend. Nevertheless, it is better to expiate your oath and attend, due to the good, kinship ties, and affection involved in it.