What is the ruling regarding a brother's oath not to hold an 'usbu' (celebration for a newborn) for his daughter, given that he is angry and his wife is still in her postpartum period, while the family desires to hold it?

The 'aqiqah is a confirmed Sunnah for the father to perform on behalf of his newborn, whether male or female. Your brother must expiate his oath and perform the 'aqiqah for his daughter, based on the saying of the Prophet, peace be upon him: "Whoever takes an oath and then sees something better than it, let him expiate his oath and do that which is better." The 'aqiqah is valid if the father gives permission for someone else to perform it. There is no breaking of the oath if he intended by his oath not to personally slaughter the animal. It is permissible for someone other than the father to perform the 'aqiqah if he refrains from it or if it becomes impossible, such as due to death. The 'aqiqah is not missed by delaying it beyond the seventh day; rather, it can be performed at any time according to the majority of scholars. The oath of an angry person is not binding if his anger is so severe that he does not know what he is saying.