OK. So, you define incest as sexual relations between brother and sister? By so defining incest, you are denying that father-daughter sexual relations are incest, since father-daughter sexual relations are not brother-sister sexual relations. Got it. Not an enviable position for you to be in.
Tell us, Arthur Brain: Why do you define incest in such a way so as to exclude father-daughter sexual relations, aunt-nephew sexual relations, sexual relations between first cousins, and other things from being incest?