How does the wife of his noncontemporaneous brother [release her co-wives]? [There were] two brothers, one of whom died, and [then another] brother was born; subsequently, the second one performed yibum with his brother's widow and died —- the first is discharged as the wife of his noncontemporaneous brother, and the second as her co-wife. [If] he performed maamar with her and then died —- the second requires chalitzah but may not be taken in yibum.

[If there were] two brothers, and one of them died, and the second [then] performed yibum with his brother's widow; subsequently, another brother was born, and then he died —- the first is discharged as the wife of his non-contemporaneous brother, and the second as her co-wife.  [If] he performed maamar with her and [then] died —- the second requires chalitzah but may not be taken in yibum. R' Shimon says: He may perform yibum with whichever of them he wishes, or perform chalitzah with whichever of them he wishes.

A general principle was stated concerning the yevamah: Anyone prohibited because of ervah neither performs chalitzah nor is taken in yibum. [If] her prohibition is by commandment or because of sanctity, she requires chalitzah but may not be taken in yibum.  [If] her sister is her [fellow] yevamah, she may either perform chalitzah or be taken in yibum.

Prohibitions by commandment [are] —- secondary [arayos] by Rabbinical decree. Prohibitions because of sanctity [are] —- a widow to a Kohen Gadol; a divorcee or a chalutzah to an ordinary Kohen; a mamzeress or Nesinite to a Yisrael; and the daughter of a Yisrael to a Nesinite or mamzer.