Yevamot 8 - Brother's wife or wife's sister?
Earlier we said that if one's brother dies childless, one has to marry his brother's widow, or release her though the procedure of "chalitzah." What if she happens to also be his wife's sister? That is, what if two brothers were married to two sisters, and one brother died. Normally we would say that a man can't marry his wife's sister while his wife is alive . However, perhaps here, since one prohibition is lifted and he has to marry the widow - then he can do so even though he then will be married to sisters?
Where would we see that "once a prohibition is lifted, other prohibitions are lifted as well?" - In the laws of a metzora (spiritual leper). Normally, a metzora is not allowed to visit the Temple. However, in order to purify himself, he needs to. And, once he is on the way to do so, he is allowed to disregard other kinds of impurity, such as last night's seminal emission. Should we apply this principle and permit one to marry any close relative if she happens to be in the "yibum" situation with him? - No! To prevent such possible way of thinking, the Torah added extra words, " to her " - to teach that the wife's sister is prohibited even in the situation of yibum.
Even though the principle of "once a prohibition is lifted, other prohibitions are lifted as well" does not apply here, because of the extra words "to her" and also because of other ways of how this prohibition can be derived, it nevertheless finds applications in other areas.
Art: Portrait of the Shishmariov Sisters by Jules Elie Delauney