Board Thread:General Discussion/@comment-14407885-20131101002935/@comment-24400457-20140422210956

Honestly I've always worked under the assumption that Yang knows something about the death of Ruby's (and possibly her) mother (as detailed in Red Like Roses Part II). Whether or not she believes Summer to still be alive (Yellow trailer?) or knows that Summer's death (if RLR2 is to be believed) was indeed to protect Ruby, and possibly herself, is something I'm still undecided about.

Her matriarchal behaviour was born of needing to be the female role model to Ruby; her happy-go-lucky, boisterous nature was born out of a need to always appear strong in front of Ruby (because how could she tell Ruby not to cry if Ruby saw her crying); and her motivations to become a huntress are embedded in a deep-seated desire to protect Ruby with possible hidden hopes of avenging/finding Summer all of which (should Yang and Ruby share a mother) could have been sparked during their final goodbye.

Of course that is purely head canon so take from it what you will and ridicule that which you desire to.