User blog comment:ChishioKunrin/Grimm-mask Samurai Woman = Yang's Mother/@comment-25091485-20141024104948

Here's another point to support your theory. The sound.

The sound directors in RWBY are able to create music/sound that suits the scene really really well.

When Raven / Vermillion / Yang's Mom makes an entrance its all epic and ominous music, and when Neo's out of the way and our mysterious Samurai looks at Yang, it switches to a soft, warming even motherly tune. Generally sound is a big part of foreshadowing intentions and that kind of stuff (like Cinder's creepy needle scene)

Also the fact that she seemed really reluctant to leave, she was staring at Yang the whole time after dealing with Neo, slowing backing away while looking at her. Then there's Yang waking up and seeing her disappear through the portal. Plot reasons aside, it seems really symbolic, like how she left Yang all those years ago.