How should a girl act after the young man's father refused her engagement to him, despite their love and despite the father seeing no flaw in her other than his desire to marry his son to his paternal aunt's daughter?

Marriage is commanded by Islamic law and achieves religious and worldly benefits, and it is one of Allah's signs. If a young man wishes to marry a girl and his father refuses, he must obey his father because obedience to him is obligatory, whereas marrying this particular girl is not obligatory. It is more appropriate for the young man to strive to convince his father so that both the marriage and the father's رضا (contentment/approval) are achieved.