Is the marriage of a brother to a woman with the intention of divorcing her so she can return to her former husband permissible in Islamic law, and is the child resulting from this marriage legitimate, especially if the child was attributed to her first husband?

The brother has fallen into grave sins due to his relationship with the woman and his marriage to her with the intention of making her lawful for her first husband. The Prophet, peace and blessings be upon him, cursed the one who makes a woman lawful (al-muhallil) and the one for whom she is made lawful (al-muhallal lahu). He must promptly offer sincere repentance and increase his acts of obedience. If the condition of making her lawful was not mentioned in the contract, then his marriage is unlawful, but the woman becomes lawful for her first husband if he has consummated the marriage with her.

As for the child, if her first husband married her after the expiration of her waiting period (iddah (definition)), then the child belongs to him. However, if she returned to her first husband before the expiration of her waiting period from the brother, then his marriage is void. This is a complex issue and should be presented to the Sharia (definition) courts for a ruling.