WE LEARNED ON THIS DAF THAT IN THE CASE OF RISHA BAR RISHA THE MAN WILL BE CHAYAV FOR 3 LAAVIM ( R' AKIVA AND R' YISHMAEL LEARN IT FROM THE POSUK OF ACHOSO AND R' YITZCHOK FROM THE KAL VACHOMER)EVEN THOUGH ITS "GOOFO ACHAS."
HOW MANY LAAVIM IS A PERSON CHAYAV FOR IF HE HAS RELATIONS WITH HIS REAL SISTER (FROM BOTH FATHER AND MOTHER)- IS IT ONE OR IS IT THREE SINCE THE GIRL IS YOUR SISTER FROM YOUR FATHER, YOUR MOTHER AND BOTH OF THEM?
PLEASE LET ME KNOW. THANKS.
ARI, NEW YORK, USA
Tosfos in Yevamos (2b) explains that a full sister is not included in the Isur of a half sister. Had the Torah not written a full sister, we would have learned it from a Kal va'Chomer, but it is not included in the Isur itself. Therefore, a full sister is one Lav. (See also Yevamos 22a-b and Makos 2b.)