1,860. Concluding Hilchos Geirushin and Beginning Hilchos Yibum v'Chalitzah

Hilchos Geirushin 13:29

Don’t be surprised that the Sages were lenient in matters involving a married woman, which is a very severe prohibition, relying on the testimony of a woman, or a male or female servant, as well as on statements made by a non-Jew in the course of conversation, written documents and witnesses who were not subject to normal interrogation, as we have explained. This is because the Torah only requires the testimony of two witnesses and all the other details of the laws of testimony when it comes to things that can only be established through the testimony of witnesses, such as that one person killed another or that one person lent money to another. However, if a matter can be established without the testimony of a witness, and a witness couldn’t justify making a false statement as is the case when someone testifies that a certain person died, then the Torah doesn’t require that the usual laws of testimony be followed. This is because someone giving a false testimony is unlikely. Because of this, the Sages were lenient in this matter and accepted testimony from a single witness based on the testimony of a servant, from a document or that was not vetted via the usual interrogation. This is so that Jewish women should not be forced to remain unmarried.

Yibum v’Chalitzah 1:1

It is a mitzvah from the Torah for a man to marry the widow of a paternal brother if the latter dies without children, as per Deuteronomy 25:5, “(if he dies) without offspring…her husband’s brother shall go to her.” This applies to both a widow from full marriage and from betrothal (which is a form of marriage). The Torah doesn’t require that the surviving brother betroth his sister-in-law (called a yevama) because she is a wife that Heaven acquired for him; he only need consummate the relationship. The deceased’s estate is responsible for the value of her kesubah.