Swearing that one's wife will be unlawful to him if his sisters go out for study

Q: I have sisters who go out for study. Once I took an oath that they would not go out for study anymore. However, they went out for study the following year. It should be noted that when I swore I said: "My wife will be Haram (unlawful) for me if you go out for study anymore." I hope you will answer my question. Have I committed a sin this way? Please enlighten me.

A: If by saying, "My wife will be Haram for me", you meant that you will divorce your wife if they went out, it will be considered a divorce and it is permissible for you to take your wife back in marriage during or after the expiration of her `Iddah (woman's prescribed waiting period after divorce or widowhood) by concluding a new marriage contract if you have not divorced her twice before. However, if you just said it to prevent your sisters from going out, its ruling is the same as that of a broken oath, so you must make Kaffarah (expiation) for an oath; which is manumitting a Muslim slave, feeding ten needy people each getting a half of Sa` (1 Sa`=3 kg. Approx.) of wheat, rice, dried dates or the common food of the country, or providing them with clothes. If you are unable to afford any of these, you must fast for three days. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.