Board Thread:News and Announcements/@comment-6533159-20151015224111/@comment-5414218-20151016204656

Busterleader92 wrote: Hey Phantom, since Qrow and Raven have the same last name, you don't think they're like brothers or sisters or something? This has been said several times already.

And yes, it's fair to assume that with the last name, the fact that Raven in Yang's mother and the fact that Yang calls him "uncle", it's almost confirmed that they are sisters. Wether blood-related or not it's impossible to tell, but it doesn't matter anyway.