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.