Yevamos 13b - Is the Mitzvah Dependent on the Issur?

I assumed that the mitzvah of yibbum depended on the situation of eishas ach, but not on the issur of eishes ach. It just so happens that in the situation of eishas ach there is also an issur. However, Rashi explains that acc. to beis shamai who is matir the tzaras ervah in yibum bec. the issur of "aishes ach" isn't chal on the preexisting issur of a daughter. Since the issur of "aishes ach" is not chal on the ervah, the ervah doesn't fall b'yibum and it is as if she is not here at all so she does not exempt her tzarah from yibum. Clearly Rashi explains that the mitzvah of yibum isn't just dependent on the situation of being married to a brothers wife, but is dependent on there actually being an issur of aishes ach. This sheds new light on the concept of mitzvaso bekach as it applies to yibum. The torah specifically gave this mitzvah only in a situation of issur. Similarly we can explain by kelayim in bigdei kehuna - it is not just coincidental that the torah wanted the kohens clothing to be made from wool and linen which happen to be an issur of kelayim. Rather the mitzvah of bigdei kehuna is to wear clothes that are a violation of the issur kelyaim.