1. Question: Why does the Gemara in Yevamos find fault in nullifying the mitzvah of Yibum if one creates a situation where it is forbidden for it to be done but if that is the case it should be forbidden to take off your four corner garment because one is nullifying the mitzvah of tzitzis?
A. The Gemara in Yevamos says that there is a view who holds that it is forbidden to nullify the mitzvah of yibum for example: If there are four brothers, two of them married two sisters and died and we say that the other two brothers should do chalitza not yibum for if one of the remaining brothers do yibum and then dies then the last brother cannot do yibum or chalitza after he die yibum to one of the sisters before because the Yevama would be the sister of his wife.
B. The mitzvah of tzitzis is to don tzitzis on a four cornered garment one is wearing or wrapped in.
C. The mitzvah of yibum is to marry the wife of your childless brother with the intention of upholding his name by having a child through his widowed wife.
Answer: The mitzvah of yibum is still there his brother still died childless it is just that he can’t fulfill it because it would be his wife’s sister who is forbidden to him, so there would be an overriding exemption. The Gemara is saying that one is not allowed to put in himself into a position that he would be forced to be exempt from the mitzvah. Whereas when one takes off his garment the entire mitzvah is gone it’s not like it is there but there is an exemption therefore it is not considered nullifying the mitzvah to take off one’s tzitzis garment.