User blog comment:DiegoAnime3000/Why they have different last names? Answer found here./@comment-14640999-20140725044458

"Yang got her last name, Xiao Long, from her father and so did Ruby. However, after Summer’s death, Ruby chose to change her own name and have her mother’s last name, therefore, Ruby Rose. And that’s why they have different last names. "

Uhh... wouldn't that make them full sisters or did Ruby get hers from a different father...? (possible issue with wording but ii see your point)