Yevamos, Chapter 1, Mishnah 3. In the two previous mishnayos we learned that if a man's brother was married to a woman who is forbidden to him as an ervah, and that brother died childless, not only is the ervah herself exempt from chalitzah and from yibum, but her co-wife, the second woman, is also exempt from chalitzah and from yibum.
Our mishnah lists six arayos that can never enter this scenario at all, since there is no way for them to be married to a man's brother: they are forbidden as an ervah to the brother who died just as they are forbidden to the surviving brother, and therefore a situation of yibum can never arise with them.
It follows that if such an ervah married an unrelated man and he died, her co-wife is permitted to the man to whom the first woman is an ervah. There is nothing wrong with marrying the co-wife of an ervah, unless both of them were married to his brother and would otherwise have been permitted through yibum, and they became forbidden because one of them is an ervah. But here, where there is no yibum at all, we are dealing with nothing more than the co-wife of an ervah in the ordinary sense, and the co-wife of an ervah is permitted. This is the point of our mishnah.
The text of the Mishnah:
"Shesh arayos chamuros me'eilu, lefi shenesu'os la'acherim - tzaroseihen mutaros" - six arayos are more stringent than these, and since they are married to others, their co-wives are permitted.
"Shesh arayos chamuros me'eilu" - they are more stringent than the fifteen arayos listed earlier.
"Lefi shenesu'os la'acherim" - they can only be married to other men, since these arayos are forbidden to both brothers alike.
"Tzaroseihen mutaros" - since their marriage is not to the brother but to an unrelated man, the co-wife does not come before us through any bond of yibum, and a man is permitted to the co-wife of a woman who is an ervah to him.
And who are the six arayos?
Imo (his mother) - a man's mother is always forbidden to his brother as well, since brothers for the purposes of yibum share one father. Even if she is not his brother's mother, she nevertheless had relations with his father and bore him, and at the very least she was violated by his father: if she was married to his father she is his father's wife, and if not, she is a woman violated by his father. Our mishnah therefore holds that a woman violated by one's father is forbidden to the son, and consequently it is impossible for the mother of one brother to be married to his brother.
Eishes aviv (his father's wife) - if his father was married to a woman and she bore one brother and afterwards another brother, the father's wife is forbidden to both of them.
Achos aviv (his father's sister) - the father's sister of one brother is also the father's sister of the other, since they have one father, and she is therefore forbidden to both of them.
Achoso me'aviv (his sister from his father) - yibum applies to brothers from the father, and if a man's sister is from his father, she cannot possibly be married to his brother from his father, since they are brother and sister.
Eishes achi aviv (his father's brother's wife) - the father's brother is shared by both brothers, and therefore his wife is forbidden to both of them.
Eishes achiv me'aviv (the wife of his brother from his father) - the wife of a brother with whom they share a father is likewise forbidden to both of them, and she cannot possibly be married to his brother.
In summary: these six arayos are more stringent than the other arayos, since they are never married to a man's brother but only to other men. Because there is no bond of yibum with them at all, their co-wife is nothing more than the co-wife of an ervah in the ordinary sense, and therefore their co-wives are permitted.