A condition whereby one has incestuous feelings towards one's sister. Mostly prevelant in areas where there is a high preponderance of ginger haired people with webbed feet and one eye in the middle of the forehead (e.g Bedford or Luton!)
Stanley "Dad! I fancy our Mable, does that make meincesbian
Dad: "No my little slack jawed, banjo playing, bedfordian offspring, it just means you have adelphepothia, same as your mother and her brother before her."