Is expiation obligatory upon one who swore not to forgive his sister and invoked (evil) upon her, then wished to forgive her, knowing that she was not criticizing anything that displeases Allah?

We advise you to forgive your sister and expiate your oath. For one who swears an oath and then sees something better than it, the prescribed action is to do what is better and expiate their oath. The expiation for an oath is feeding ten needy people (approximately 1.5 kg of rice for each needy person), or clothing them, or freeing a slave. If you are unable to do that, then fast for three days, and it is more cautious for them to be consecutive. We advise both of you that your relationship should be based on affection, respect, and advising each other with what is best.