A girl who is very attractive but due to more attractive sister(s) or long term friends has not gotten exposed to people thinking she's hot since they all go for the sister or friend. Usually consider themselves ugly and like in Ugly Duckling Syndrome have good personalities and will date any guy because "Looks aren't important" and they have a good sense of humor.
Originates from Cinderella.
Aaron: Amy you're hot
Amy: No I'm not, my sisters are and I got the shitty genes
Aaron: Want to come over later?
Amy: Sure, your funny.
Aaron: Hurray for Step-Sister Syndrome