Deutéronome 25:5

When brethren from the (same) father inhabit this world at the same time, and have the same inheritance, the wife of one of them, who may have died, shall not go forth into the street to marry a stranger; her brother-in-law shall go to her, and take her to wife, and become her husband.

They also claim that the phrase when brothers dwell together denotes close relatives, but not actual brothers. They bring the case of Boaz [cf. Ruth 3: 9 ; Ruth 3: 13] as evidence. This is vacuous, however, because no mention is made there of the word yibbum [here translated as “acting as the husband’s brother” — Translator], only of “redemption” [cf. Leviticus 25: 25]. Besides, what, then, is the meaning of the word together ? That they live in the same country? In the same courtyard? That they are fond of each other? The Dissenters claim further that the Scriptural usage of the term act as her husband’s brother proves that she is his brother’s wife in name only, but not in fact, as in the phrase “your brother’s wife has returned” [Ruth 1: 16]. But they have stumbled into error. Note that regarding the sons of Y ǝ hudah, Scripture explicitly says, “act as her husband’s brother” [Genesis 38: 8], although there he clearly already was her husband’s brother. Even the abovementioned use of the word “your brother’s wife” can be understood on the grounds that the two women were married to two brothers. Every sensible person knows that, although the prohibitions that were told to Moshe were not forbidden beforehand, if someone before the time of Moshe refrained from engaging in such actions he would not have displeased God . Scripture even states, “for the men of the land who preceded you practiced all these abominations” [Leviticus 18: 27], and yet we have no evidence that any prophet had ever admonished them against such actions. With respect to Onan Scripture specifically says that he did not “provide progeny for his brother” [Genesis 38: 9] and consequently “it was evil in the sight of God ” [Genesis 38: 10]. For these reasons, we will rely on the tradition that actual brothers are meant here.

childless For there is no need to perform the ritual [cf.: 7 ff] if he had a child, since the widow is then forbidden [Leviticus 18: 16].

together is to be understood as the Exegetes have spoken [Yevamot 17b].

Even if the husband’s brother consents, she may not marry outside (i.e., a strange man).

ויבמה, by doing so he removes her status as yevamah from her as she is now his wife. She will be a wife in all respects including the legislation applying to divorcing her and possibly remarrying her.