What is the ruling on swearing by divorce not to marry one's sister to a specific person, and is expiation required if the marriage takes place?

An oath by divorce has two situations: 1. If the one swearing by divorce intends for the divorce to occur upon the fulfillment of the sworn-upon condition, then the divorce takes place. 2. If the one swearing does not intend divorce but rather intends to prohibit or deter, then scholars have differed: The majority of scholars hold that the divorce takes place upon the fulfillment of the sworn-upon condition. Shaykh al-Islam Ibn Taymiyyah holds that this is akin to an oath, and an expiation for an oath (kaffarat yameen) becomes obligatory upon the fulfillment of the sworn-upon condition.

The questioner is advised to fear Allah, to avoid swearing by other than Allah, and to refer to the Sharia court to clarify the matter.