Board Thread:News and Announcements/@comment-6533159-20151015224111/@comment-14909251-20151020233702

MetallicArcher wrote:

Blake knows since before the forming of RWBY that Ruby and Yang are sisters, and she never gave signs off finding the different surnames odd. Moreover, what actually surprised her was learning they were half sisters. Weiss is also aware they are sisters and she hasn't questioned the different surnames.

It's perfectly possible that in Vale's culture children may be given whoever parent's name. Maybe it wasn't a matter of not finding it odd and more a matter of not being intrusive. Blake isn't exactly the most outgoing person and neither is Weiss. Never mind that both may have some sense of it being rude to ask.