What should a young man do who broke off his engagement to his paternal uncle's daughter, and now wishes to return to her despite the refusal of his father and his uncle, and what is the advice for parents in this situation?

You must repent and regret what you did with your cousin and cut off the relationship with her until a legitimate marriage takes place. You must also repent for offending your uncle, for "abusing a Muslim is فسوق (disobedience), and fighting him is kufr (disbelief)."

You have two options: either marry your cousin and adhere to the manners of Islam, or turn away from her and conceal what happened between you.

Your father is advised to help you marry the one you love and to forgive past disagreements between him and his brother, for "those who restrain anger and pardon people—and Allah loves the doers of good," and maintaining kinship ties is obligatory.