Is the statement of a relative that he will marry his daughter to the questioner's son, while the son is still young, legally binding? Does the relative have the right to marry his daughter to the questioner's son now? And does the questioner have the right to demand this marriage?

It appears that the phrase "my close relative" was a promise of marriage and not a legally binding contract. Even if a contract was intended, it would not be valid according to many jurists. Therefore, if you wish to marry her, you must enter into a new contract in the presence of two witnesses. If you do not wish to marry her, or if her father does not agree, there is no harm in him marrying her to someone else. It is more prudent to divorce her as a precaution, taking into account the opinion of those who validate a contract with the aforementioned phrasing.