Is it permissible for a man to marry a woman he does not love, knowing that she dislikes him, in order to please her family and fulfill the condition for his sister's marriage to her brother?

The Prophet, peace and blessings be upon him, forbade Nikah Al-Shighar, which is a marriage where one party stipulates that the other marry their daughter, sister, or female ward, whether a dowry is given or not. Such a marriage is considered invalid. This is what happened in the marriage in question, as the questioner committed a grave religious transgression. Marriage must be based on consent and free choice, and violating this leads to marital failure. The danger lies in the questioner's attachment to the sister of the girl he married, which could lead to forbidden acts.

Therefore, the questioner is advised to avoid completing this marriage. He should explain to his brother-in-law that stipulating both marriage contracts together is forbidden and invalidates both contracts. He should adhere to his wife while re-contracting the marriage correctly, as it is invalid due to Al-Shighar. If his brother-in-law refuses this and insists on separating from her, he should remember the words of Allah Almighty: "But if they separate [by divorce], Allah will enrich each [of them] from His abundance. And ever is Allah all-Encompassing and Wise." (An-Nisa/130).

The questioner must not communicate with the girl he desires if he cannot marry her amicably, and he must completely cut ties with her.