Is it considered a sin for a fiancée to wait and pray to God for her marriage to her fiancé, whom his family forced to marry another woman?

There is no objection to waiting for a man you wish to marry, and supplicating to Allah to facilitate your marriage to him. However, it is not appropriate to supplicate for the dissolution of another woman's engagement. It is better to supplicate for a righteous spouse without specifying a person, and to entrust the matter to Allah, for Allah knows best what is good. Parents do not have the right to compel their child to marry someone they do not desire.