Board Thread:Speculation House/@comment-68.186.83.101-20130601225528/@comment-4010415-20150306100727

Beewyka819 wrote: Actually, in a tweet Monty confirmed that they ARE blood related, I assume sisters, as in Yangs flashback she has a brown or red hair color (a tad bit color blind, couldn't tell in that kind of shading) so it seems that Yang isn't naturally blonde, further increasing the chance that they're blood related, maybe the last name thing doesn't really work the same on Remnant. Wait, yup, on the Wiki it states that they're Paternal Half-Sisters This thread ended in 2014, as you can see by the post above yours. We all know now that they are related.

Also, apparently, Yang's hair really was blonde when she was a kid, but for some reason, the lighting they decided to use made it look brown. Refer to the concept art of Yang and Ruby as children for proof: