Is expiation obligatory for someone who swore not to forgive his sister and then recanted and wished to forgive her?

If one swears an oath concerning a matter and then breaks it, an expiation for the oath is due. You have acted in accordance with the Sunnah (definition) by retracting your refusal to forgive your sister, as the Prophet (peace be upon him) said: "By Allah, if Allah wills, I will not swear an oath and then see something better than it, except that I will expiate for my oath and do that which is better." The expiation for an oath is: feeding ten poor persons, or clothing them, or freeing a slave. If you cannot find any of these, then you must fast three days.