User blog comment:Lauren Darkmore/The RWBY AMA on Monty's twitter/@comment-10390252-20131111000031/@comment-5217560-20131111174525

Wait, Monty never said they weren't sisters. He's always said that they were sisters. What's perplexing everyone is that these two ladies have different last names., which isn't strange in this type of world I would imagine. And according to Ruby, both parents have been present in both of her and Yang's upbringing. So, my guess for the different last names may be due to their different characteristics. Then again, them being half-sisters is just as plausible. But I feel people are throwing that in there because we assume Summer Rose is  blood related to Ruby. Eh. I hope they clear that up in the second Volume.