User blog comment:Lauren Darkmore/The RWBY AMA on Monty's twitter/@comment-10390252-20131111000031/@comment-4010415-20131111080759

Hmm, interesting idea, Inferno. One getting lost and later found, possibly raised by another family in the meantime.

What if Ruby is Yang's niece, though, and since they're so close in age, they decided to just refer to each other as sis anyway? Lol. I actually know a 6-year-old who has a nephew who's not much younger than her.