All female relatives [that are married to someone's brother, but are forbidden to him]

for which they said one her co-wives were permitted [to marry others if their first husband dies childless, rather than falling to yibum to his brother, due to the status of their co-wife as a forbidden relative],

if these female relatives go and marry

and are found to be unable to bear children [thus voiding the marriage to the first husband]

[if her co-wife who would have fallen to yibum in other circumstances remarried], she must leave both [her current husband and her late husband's brother, via chalitzah]

and all of these measures [listed in the previous mishnah] apply to her.

One who marries his yevamah

and her co-wife goes

and marries another

and it turns out that this one [the yevamah]

is unable to bear children

she [the co-wife] must leave both [her current husband and her late husband's brother, via chalitzah]

and all of these measures [listed in the previously] apply to her.