The Masechta opens by listing close relatives with whom yibum is not allowed — including cases of eishes ach (a brother’s wife) outside the mitzvah of yibum.

This immediately raises the core question: How can yibum ever work? Yibum is, by definition, marrying an eishes ach, which is normally an issur punishable by kares.

Tosfos based on Kovetz He’aros 9

Yibum works through the rule of aseh docheh lo sa’aseh — a positive mitzvah overriding a prohibition.

The issur of eishes ach remains fully in place; it is simply overridden in this scenario.

Tosafos asks: If yibum overrides ervah, why not apply it to other forbidden relatives, like a wife's sister?

Tosafos answers: Yibum is allowed only because the Torah explicitly commands it. The override applies only here, nowhere else.

Yad Ramah’s Approach (Sanhedrin 53a):

Yibum is not based on aseh docheh lo sa’aseh.

Rather, the Torah’s prohibition of eishes ach never applied to a case where yibum is possible.

The issur applies only when the woman has children and yibum is off the table.

Ramban’s View (Toras Ha’adam):

Agrees with the Yad Ramah.

Compares it to a kohen becoming tamei for a mes mitzvah: it’s not an exception — that case simply wasn’t included in the original prohibition.

So too, eishes ach simply doesn’t apply in a scenario of yibum.

Nafka Minas (Practical Differences Between the Views):

Kiddushin with a Yevama:

Rashi says kiddushin cannot take effect, because the issur of eishes ach still exists (fits with Tosafos/Rav Elchonon).

According to Ramban/Yad Ramah, kiddushin might work, since eishes ach isn’t present at all in a yibum case.

Yibum for the Wrong Reasons:

The Gemara warns it’s like living with an ervah.

Rambam says it’s not actually an ervah at all — once the brother dies, the issur disappears.

Rambam fits with Ramban: the issur is simply gone in a yibum case.

Tosafos/Rashi fit the stricter language of the Gemara — the issur exists but is overridden.

This sugya sets up a major theme of Yevamos:

Is yibum an override of eishes ach, or is eishes ach simply not present when the Torah commands yibum?