What should I do about my promise to a woman to marry her after seeing her body, and my failure to fulfill the promise because my family chose another wife, and my fear of her supplication against me, knowing that my marriage to her would lead to my family disowning me?

What you both did is forbidden and necessitates repentance. If you see that the girl has repented and would make a suitable wife, then you may fulfill your promise and marry her, even if she is a second wife and without your family's knowledge, provided you fulfill her rights. This is not an obligation, so if you see that she would not be a suitable wife for you, then turn away from her. It is permissible to retract an oath if something better is seen, as the Messenger of Allah, peace and blessings be upon him, said: "Whoever takes an oath and then sees something better than it, let him do what is better and make expiation for his oath." You are required to perform an expiation for the oath, which is: feeding ten poor people, or clothing them, or freeing a slave. If you cannot find any of these, then fasting for three days. It is not permissible to move to fasting if you are able to perform one of the first three.