User blog comment:EvilAngel5/RWBY Character Mysteries/@comment-7472785-20130807054803

"Assuming that Ruby was adopted, (and I'm pretty sure she was)..."

Why?

Couldn't as much Yang be the adopted one? If any of the two is adopted that is.

In the 3rd episode they mention their dad - in a way that lets assume he's still around and a salient feature in their life so far - but they could have different mothers.

Or Yang comes after their mother and Ruby more after their father - or vice versa.

Adoption is but a hypothesis, and IMHO not the most probable.