When brothers dwell together and one of them dies without a son, the widow must not marry outside the family. Her husband’s brother is to take her as his wife and fulfill the duty of a brother-in-law for her.
5. If brethren dwell together, and one of them die. This law has some similarity with that which permits a betrothed person to return to the wife, whom he has not yet taken; since the object of both is to preserve to every man what he possesses, so that he may not be obliged to leave it to strangers, but that he may have heirs...
Here is, I. The law settled concerning the marrying of the brother's widow. It appears from the story of Judah's family that this had been an ancient usage (Gen 38:8), for the keeping up of distinct families. The case put is a case that often happens, of a man's dying without issue, it may be in the prime of his time, soon after his marriage...
Commenting on Deuteronomy 25:5-12
If brethren dwell together,.... Not only in the same country, province, town, or city, but in the same house; such who had been from their youth brought up together in their father's house, and now one of them being married, as the case put supposes, they that were unmarried might live with him, and especially if the father was dead; and so may except such...