Is expiation (kaffarah) obligatory for one who swore an oath not to harm his siblings, but then broke his oath without harming them?

If an oath is contingent upon a specific event and that event occurs, then it is a binding oath. The obligation is either to fulfill it or to break it and offer expiation for it. If fulfilling the oath causes harm or distress to the children, then it is preferable to break the oath and offer expiation. The expiation for an oath is to feed ten poor persons, or clothe them, or free a slave. Whoever cannot find the means to do so, then he must fast for three days.