User blog comment:WeissSchnee/Character Blog: Yang Edition/@comment-7472785-20130621073504

"I for one think they’re adoptive sisters, with Yang’s family having adopted Ruby when both the girls were very young."

As a matter of principle - nerdy, I concede - it could as well be Ruby's family adopting Yang, for all we know they could also both be adopted.

Other 'sis' provoking circumstances could be:

- membership in a sorority

- colloquial habits

- just old friendship

Or, nothing of the kind - as you already said, we'll have to wait and see.