Yevamos, Chapter 3, Mishnah 3. This mishnah deals with a case very similar to the one in the previous mishnah, except that here it is doubled: the very same idea occurring twice.
The case before us:
"Haysah achas mehen asurah al zeh issur ervah" - there are four brothers, two of whom are married to two sisters. The two brothers married to the sisters died, and their wives fall to yibum before the two remaining brothers. One of the two sisters is forbidden to one of the brothers as an ervah - let us say she is his mother-in-law.
"V'hashniyah asurah al zeh issur ervah" - the second sister is forbidden as an ervah in her own right to the other brother; she too is his mother-in-law.
"Ha'asurah la'zeh mutteres la'zeh, v'ha'asurah la'zeh mutteres la'zeh" - in such a case the question resolves itself:
The sister who is an ervah to the one brother is permitted to the second brother.
The sister who is an ervah to the second brother is permitted to the first brother.
"V'zu hi she'amru":
The continuation of the mishnah is simply a repetition of what was explained in the previous mishnah, except that here it happens twice. The reference is to the rule stated in the second chapter, in the case where two sisters are married to two brothers: "Achosah k'shehi yevimtah - o choletzes o misyabemes" - when the sister of the ervah is his yevamah, that is, when the sisters are married to the brothers, the option is open either to do chalitzah or to do yibum.
The reason: this is not a case of "a sister and her sister," since one of them is an ervah to the one brother and the other is an ervah to the other brother, and each one of them is permitted to the brother to whom she is not an ervah. Therefore, in such a case both of them may undergo yibum or chalitzah.