Yevamos 2:3-4

Yevamos 2:3

The general rule about a yevama is that when a marriage is a Biblically-prohibited relationship, they perform neither yibum nor chalitzah. When a marriage is a “mitzvah” prohibition or a “sanctity” prohibition (as will be explained in the next mishna), then they perform chalitzah but not yibum. If the sister of a prohibited woman is her sister-in-law, he may perform yibum or chalitzah with the non-prohibited woman.

Yevamos 2:4

A “mitzvah” prohibition includes women who are not prohibited by the Torah but who were prohibited by the Scribes as secondary relations (e.g., one’s son’s daughter-in-law). A “sanctity” prohibition includes the Kohein Gadol not marrying a widow, a regular kohein not marrying a divorcee or a woman who performed chalitzah, a regular Jew not marrying the product of an incestuous or adulterous relationship (mamzeres) or a Gibeonite (see Joshua chapter 9), and the daughter of a regular Jew not marrying a mamzer or a Gibeonite.