In the 1800's, father-daughter marriage was not illegal in most of the US.

Brothers and sisters are forbidden from marrying each other by law because God said so but apparently God doesn't know much about the actual legality of such unions.<br><br>Brothers and sisters are forbidden from marrying each other by law because of science but science clearly doesn't know much about the actual legality of such unions.<br><br>Brothers and sisters are forbidden from marrying each other by law because of sociological norms but apparently such norms aren't common enough to extend to father-daughter marriage.<br><br>Brothers and sisters are forbidden from marrying each other by law because of referential statistics (no referential statistics for father-daughter referential statistics).<br><br>Brothers and sisters are forbidden from marrying each other by law because they might love each other in the wrong way or something.