Board Thread:News and Announcements/@comment-25148498-20141031183601/@comment-4010415-20141031184521

Both Ruby and Yang refer to Qrow as their uncle, but their father's name is Taiyang Xiao Long.

But then, you consider the fact that Yang grew up thinking Summer was her mother, only to find out the truth after Summer's death/disappearance, and Summer's last name is Rose, not Branwen.

I think Qrow is Yang's uncle by blood, and is only considered Ruby's uncle by close relationship and familiarity.