The gemara says (23b I believe) that if not for the rule 'ain mazhirin min hadin', we would learn out the issur of 'sister from mother and father' from the issur of 'sister from the father' by a kal v'chomer.
My question is that this doesn't really seem to be a kal v'chomer, but rather 'biclall mosayim mana', included in the prohibition of achoso. For isn't a sister from the father and mother automatically a sister from the father?