The Torah in Parashat Ahareh-Mot (Vayikra 18:15) commands that one may not engage in intimate relations with “Eshet Ahicha” – one’s brother’s wife. This applies regardless of whether the person and his brother share both parents, only a father, or only a mother. Additionally, even if the brother merely betrothed a woman, and had yet to complete the marriage, one is forbidden from engaging in relations with the bride, forever. Even if the brother subsequently dies or divorces his wife, she remains forbidden for the brother-in-law. The famous exception to this rule is a situation of Yibum – where a married man dies without any children. In such a case, it actually becomes a Misva for the deceased’s brother to marry the widow. Whereas generally a sister-in-law is forbidden for marriage even after the brother’s death, in this instance, there is a Misva for the brother to marry the widow. This command applies in all times and in all places. If one had relations with his brother’s wife while the brother was still married to her, then they are liable to Henek (execution by strangulation) for violating the prohibition of Eshet Ish (adultery). If this encounter occurred after the brother had died or divorced the woman, then the violator is liable to Karet for transgressing the prohibition of relations with one’s sister-in-law. If one committed this transgression mistakenly, he must bring a Hatat (sin-offering). It is worth reflecting on the fact that a relationship which is generally strictly forbidden, and regarded as a capital offense, can become not only allowed, but a Misva, in certain circumstances. The institution of Yibum, which designates as a Misva a marriage between a man and his childless brother’s widow – a marriage which would otherwise be forbidden – shows us how we must humble ourselves before the Torah’s authority. Only G-d Himself determines when something is permitted, forbidden, or a Misva. We are to humbly submit to the Torah’s commands, without ever thinking that we can intuitively decide what Hashem wants of us.