Is the marriage of a daughter to another man considered null and void after an agreement was reached for her marriage to the questioner's brother and the news spread among people, then her father married her to others?

If the brother has entered into a valid, Sharia (definition)-compliant marriage contract with the woman, fulfilling all its pillars and conditions, then she has become his wife. Her marriage to another man would then be null and forbidden. Ibn Qudamah stated: "As for invalid marriages, such as the marriage of a woman who is already married or undergoing her waiting period ('iddah (definition)), or similar situations, if they both know the permissibility and impermissibility, then they are adulterers and subject to the prescribed punishment, and lineage is not established through it."

However, if the intention is that the man accepted the proposal of marriage but did not formalize the marriage contract, and then her father married her to another man, the marriage is valid. But the second suitor would have committed a forbidden act if he was aware of the first man's proposal. Ibn Qudamah stated: "A man proposing marriage when his brother has already proposed is forbidden in the context of the prohibition... If he does so, his marriage is valid, as Ahmad explicitly stated, saying that they should not be separated."