I have a sister. We are close in age and for a while we went to a school with a uniform. People would occasionally ask us if we were related. I figured it was becuase we were together and wearing similar clothes, because I look just like my mom, and my sister looks a lot like my dad's side of the family. (People do not seem to ask us this when we are not dressed alike.)
At one point two separate people (in different countries even) told me I looked just like that girl on "Blossom".
But recently I have a friend, and the two of us get asked constantly if we are sisters. It's funny, because I don't think we look terribly alike, and in fact could give you a longer list of things I thought were different (such as our heights, body types, hair color, eye color...), but enough people have asked over several years, so there must something there. My friend is actually getting a bit annoyed. It's especially funny because people seem a little disbelieving and ask if we are maybe cousins or something.
But this weekend I was at a (different) friend's party and someone asked if we were related, and while we are not, we both looked at each other and agreed that the question didn't seem surprising. Now, I probably don't look that much more like the second friend, but for some reason, that makes more sense to me that people would think we might be related. Which just goes to show, as highly developed as human facial recognition is, it's still all very subjective.