How can people tell we're sisters??

My daughters seem surprised when someone looks at them within a group of friends and picks the two of them out as sisters.  Although they are only 18 months apart in age, they are very different in height (Maea is 5'1" and Emme is 5'6") and very different in personality (one is more of an extrovert and one is more of an introvert). 

And yet perfect strangers can pick the two of them out of a crowd and ask, "Are you sisters?" 

I have explained to them how there are subtle similarities that show up even in their actions and the things they do ....

Two examples in photo form - One taken in 2001 on the beach in Florida when they were little tiny girls and one taken 10 years later (November 2011) in Ethiopia.