But we are not out of the woods yet!!
Why is a צרת ערוה forbidden? It is NOT because the ערוה forbids her צרה but rather because she makes her צרה to be פטורה מן היבום at which point she then becomes אסורה because of אשת אח [as emerges from Rashi and Tosfos 3a]. One is not allowed to marry his late brother's wife unless there is a מצוה of יבום. If there is a פטור יבום then we revert back to the original איסור אשת אח. So a צרת ערוה is פטורה מן היבום resulting in an איסור אשת אח. If so, in our case, if the צרה of the ערוה ["Dinah"] is אסורה because of אשת אח then it makes sense that her צרה ["Sarah"] should similarly be forbidden as the צרה of an ערוה.
So Tosfos may be correct that we can't forbid Sarah as the צרת צרה of the בועל [because there is no איסור of צרת צרה for the בעל and the בועל can't be more stringent] but let's forbid her as the צרה Of an אשת אח שלא במקום מצוה which spells ערוה.
