An F-Marry-Kill Conundrum

Late in the show, a caller presented Howard with what may be just be the most difficult F-Marry-Kill he has ever had to consider:

Robin, his sister Ellen and his mother-in-law.

Howard decided he would have to F Robin, and thought his wife would leave him if he killed her mother but was also worried about his own mother's reaction if killed his sister. Howard finally decided that he could not fulfill the caller's request and refused to answer.