The gemara teaches that we can't apply a kal vachomer to malkut and to issourim when there is a passouk that indeed teaches just that (ref: a daughter of a father or a dahghter of a mother). In this week's parasha ki tavo 27.22 it seems that one can curse one's sister of both father and mother, since we can't apply a kal vachomer. Or we can say that since ther is no specific passouk for a sister of both a father and a mother ( in passok 27.22) we could apply the kal vachomer and say that if one cannot have relations with a sister of a father or of a mother then certainly one cannot have relations with a full sister. What do you say?

Nissim Bitton, Montreal, Canada