User blog comment:Angren Túrë/How do you react?/@comment-13499046-20130916201908

What if neither is adopted? Or both? They could be half sisters, or step sisters. In fact, we don't even know if surnames work in the RWBY world the way they do for us. They could be proper sisters who each grew up with a differant parent etc. etc.

Or perhaps Ruby's parents weren't married when she was concieved, and she was raised by her mother (white cloak/summer rose) because, for whatever reason, her father left and went on to marry Yang's mother. But when Summer/White Cloak went of on Huntress-business (and maybe didn't come back?), she then lived with her father and Yang's mother, and developed a strong sisterly bond with her.