User blog comment:FinalWholockianGrrlGamer/Headcanon: Summer Rose/@comment-10390252-20131216165227

My theory:

Summer Rose is Ruby's mother and died when Ruby was fairly young. Their father was distraught and temporarily unable to care for his daughters so Yang stepped up and played a big role in Ruby's upbringing, reading her stories at bedtime and the like. It's one of the reasons why Ruby is so fixated on Yang; she's more of a mother figure in her subcinscious.

There remains a considerable mystery as to how and why Summer died on that oh-so-secret mission; the most the girls know is what their father told them - that it was vital for the future of remnant. Yang wants to know the whole truth and that is why, in the Yellow Trailer, we saw her chasing the only surviving member of Summer's team, their maternal aunt, Autumn Rose.

What Yang doesn't know is that Autumn knows a lot about what happened on that mission and it has twisted her into something completely different from the heroic huntress that Yang remembers. These days, she's known as Cinder Fall.

Regarding names, I think that Yang is using her father's surname whilst Ruby, either by traditional convention or personal choice, is using their mother's maiden surname.