Does a divorce take effect if the husband sees his wife's brother at her family's house, after he swore an oath of divorce regarding that?

If you said to your wife: "If you see her brother while you are at her parents' house during their visit, then you are divorced," this is a conditional divorce. The fatwa (legal opinion) adopted by us is the view of the majority, which states that divorce occurs upon the breach of the oath, whether you intended to effect the divorce or merely to threaten. So, if you saw her brother during your visit to her family, her divorce takes effect, and you may take her back before the expiration of her iddah (waiting period), unless this divorce completes the three divorces.

As for the view of Sheikh al-Islam Ibn Taymiyyah, if you swore with the intention of threatening and did not intend to effect the divorce, then the divorce does not occur upon seeing her brother; however, you are obligated to pay an expiation for breaking an oath (kaffarat yamin). Swearing by divorce is considered an oath of the wicked, and grave consequences may result from it.