Yevamos, Chapter 3, Mishnah 2. This mishnah combines a concept we learned in the second chapter with the concept that came up in the previous mishnah.
Background - the previous mishnah:
The previous mishnah dealt with four brothers, two of whom were married to two sisters, and those two brothers died. A problem of 'achos zekukaso' (the sister of a woman bound to him) arises here: none of the brothers can perform yibum, since each of the sisters he would want to take in yibum is the sister of a woman who is bound to him - after all, both sisters fell to both brothers for yibum simultaneously. Therefore they must perform chalitzah and not yibum.
The novel point of our mishnah:
"Haysah achas mehen asurah al echad issur ervah" - one of the two sisters stands in a relationship of ervah toward one of the brothers, and not because she is a sister but for an entirely different reason, such as that she is his mother-in-law. In this case the ruling changes with respect to each of the brothers:
The brother to whom she is an ervah: He is forbidden to her because of the ervah, but he is permitted to her sister. The mother-in-law is not bound to him at all and does not fall before him for yibum, and it turns out that her sister is not 'achos zekukaso'.
The second brother: Who has no relationship of ervah with either of the sisters - he remains forbidden to both of them.
Issur mitzvah and issur kedushah:
What is the ruling when we are not dealing with a prohibition of ervah toward one of the brothers, but rather with an issur mitzvah or an issur kedushah?
Issur mitzvah: For example, that she is a sheniyah, which is a sort of rabbinic ervah.
Issur kedushah: For example, a divorcee to a kohen.
In such a case the ruling is "chotzos v'lo misyabmos" - the sisters perform chalitzah and do not undergo yibum, since on the Torah level she is still considered bound, and her sister is 'achos zekukaso'.
In summary: In this mishnah we learned that a prohibition of ervah toward one of the brothers removes the bond entirely, and therefore that brother is permitted to the sister, while the second brother remains forbidden to both of them; whereas an issur mitzvah or an issur kedushah does not remove the bond on the Torah level, and therefore the ruling is chalitzah and not yibum.