Is it a sin to pray that the man who proposed to her be her destiny, especially after her father’s strong refusal of him because his father has vitiligo?

You bear no sin for your supplication; it is not a sin or a severance of kinship ties. Rather, it is a request to Allah to heal a Muslim and to soften your father's heart so that this righteous young man may be destined for you. This aligns with what Islamic law encourages. Supplication benefits both what has descended and what has not descended. We advise you to obey your father and speak to him gently, trying to persuade him respectfully. If you think that revealing this supplication would displease your father, then there is no harm in supplicating in private.