User blog comment:Ncduru/The Erza-Irene Connection/@comment-73.191.17.103-20160430012747/@comment-1359322-20160430045033

In FT you got:
 * 1) Ancestor and descendants who looks identical (Heartfilia)
 * 2) Father & son who don't look a like (Dreyars)
 * 3) Siblings who look alike (Strauss)
 * 4) Cousins who act alike (Blendy)
 * 5) Siblings who don't look alike (Dragneel)
 * 6) Cat and human who looks and act the same (N/Ichiya)
 * 7) Mother and daughter who looks alike (Ur)

Either way, it's like anything goes here.