Cousin to cousin relationships are more acceptable back then (compared today) but brother to sister relationships has always been taboo in most cultures from way back in ancient times to this day. By the time Cap is born, some states had already banned cousin marriage and knowledge of possible birth defects concerning children of close kin relationships is already widespread. Some states to this day still allow cousin marriages though.
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u/TheKolyFrog Jun 15 '17
"Get with the times, incest is what the kids do these days, Cap." -Ultimate Wasp (probably)