Board Thread:News and Announcements/@comment-6533159-20151015224111/@comment-24994749-20151019171730

I'm not saying it's impossible for Taiyang and Summer to have not been married, I'm saying that Ruby not carrying Taiyang's surname =/= Taiyang and Summer not having been married. I would find it more logical if people justified the theory of Summer and Taiyang not being married by saying he might still be legally married to Raven. And that would be assuming marriage in Remnant works like marriage here in the West.

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.