Is the father's oath not to marry his daughter to one of his brother's daughters considered forbidden for her, and what must he do regarding this oath, especially after the daughter's desire to marry one of them after three years?

You should not proceed with an action that would displease your father, because obedience to him is obligatory. Marrying your female cousins is not obligatory. If your father agrees to your marriage to someone other than your female cousins, he must expiate his oath. You should convince him that this is the better option, based on the Prophet Muhammad's (peace be upon him) saying: "Whoever takes an oath and then finds something better than it, let him do that which is better and expiate his oath." The expiation for an oath is: feeding ten poor people, or clothing them, or freeing a slave. If he is unable to do that, then fasting for three days, based on the Almighty's saying: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or freeing a slave. But whoever cannot find [or afford it] - then a fast of three days. That is the expiation for your oaths when you have sworn. And guard your oaths. Thus does Allah make clear to you His verses that you may be grateful."