Border-line crack theory time( all evidence is iffy at best and a lot of assumptions are made). Raven Brandwen is Sienna Khan. Being that we know nothing about Khan, I will pull mostly from Qrow and Raven's little chat.
The tribe that raised Qrow and Raven was or became part of the White Fang, with Qrow's and assumedly Raven's shape-shifting abilities as their faunus traits( some form of pure-blood faunus trait as opposed to mixed-blood resulting in only a single trait maybe?). The tribe they were from, and Raven in particular, was always supporters of more violent methods than the original White Fang was using, and Raven left Taiyang to start or support the coup that would eventually oust the Belladonna's as leaders, with Raven changing her name to Sienna Khan separate herself from her time at Beacon( and hide that one time she had a kid with a human) as it could be something she doesn't want her human-hating comrades to know, while taking charge relatively recently.
As I said before, evidence iffy but here's what I got:
1) When Qrow describes their tribe as "killers and thieves" and with the attack on Chion? many people assume that they are simple bandits but "killers and thieves" is also a good description of the White Fang these days.
2) When talking about her new leadership position, Raven refers to her group as "our people" which to me sounds like a much larger and more unified than a group of bandits who presumably raid towns for supplies. The term "our people" seems more like she's referring to an entire people group as opposed to just a group of people.
3) Raven's Grimm mask. The mask is such an obvious connection to the White Fang, the only way for it not to be would be that, for what ever reason, she's coping the WF, they're coping her or it's just coincidence that they both wear Grimm masks and they have no relation to each other which would be a rather poor choice on RT's part. Also note on the WF masks, the higher the rank of the individual, the more detailed and ornate the mask as a way of displaying said rank and with Raven being the leader, having the biggest and baddest mask would make sense.
But what I like most about this theory is how it works into the show as a whole. For one, if Qrow is correct in his accusations that Raven has a maiden under her thumb, it would mean that the WF has control of a maiden, the same WF that that Salem is trying to ally herself with. More importantly, it could severely impact Yang and Blake's arc over the volume, here's what I came up with:
Blake know's Raven but only as Sienna Khan due to her time in the WF. She also knows that not only is Raven largely responsible for causing her father to step down as leader of the WF but also as Adam's mentor (or mother if you want to believe that theory) and the one who started him on and encouraged his dark nature. She doesn't know she's Yang's mother.
Yang, at the start of vol. 5, suffers from a severe PTSD episode, (just because she's out of her depression doesn't mean the PTSD is gone) possibly due to being in her first real fight sense Beacon, leaving her in a very vulnerable emotional state. Raven, who has been keeping tabs on Yang, decides now is the best time to visit her daughter and offers to train her and make her strong. Yang, who is tired of feeling weak and helpless due to her PTSD combined with the bombshell of meeting her mother for the first time on top of everything else she's going through, accepts without thought. Raven, of course, has less than charitable motives.
Through-out the volume, we see Blake come to Mistral in her fight against the WF while Raven train Yang while intentionally pushing Yang to be more like herself by encouraging her anger or eroding away what trust she has in other people, particularly in her team while never reveling that she's the leader of the WF. Over the course of the volume, we see that Raven has been at partially successful. I don't mean that Yang's casually murdering people by the end of the volume, but more Yang's still hurting from the Fall of Beacon and under her mothers influence she's starting to lash out violently, particularly at things that can or have hurt her.
Then we get to the season finally. Blake's war against the WF has gone poorly and she ends up in a losing battle against Adam alone, is beaten and is about to be captured or killed. But before Adam can do either, Raven, who has been watching them both, portals in, interrupting what Adam was doing. Both recognize her as Sienna Khan, but she quickly silences them and says she disproves of Adam's alliance with Salem and/or his obsession with Blake, saying it makes him weak. Shortly after Yang also steps out of the portal, also wearing a full Grimm mask similar to Ravens while looking very different from the last time Blake saw her, and immediately focuses all of her attention on Adam, missing Blake entirely. Blake also doesn't recognize Yang either. Raven then tells Yang she needs to kill Adam to prove her strength while Adam needs to kill Yang to get back in Raven's good graces. And while Yang has yet to kill anyone, she and Adam immediately jump into battle. Blake tries to use this distraction to escape, still unaware of Yang, but is stopped by Raven and they begin to talk while Adam and Yang continue to fight. Due to her training, Yang soon gets the upper-hand and has Adam at her mercy and looks ready to kill him but hesitates, as she still hasn't killed before. At this point, Raven intentionally reveals that she is Yang's mother and Blake is terrified by this as she finally recognizes Yang. Blake rushes over to Yang to try and talk her down from killing Adam and to stop her from following down the same path. Yang is understandably quite angry at Blake and a verbal fight brakes out between the two and Blake sees first had just how much her running away has hurt Yang. Over the course of the argument, it becomes clear that Yang is going to side with Blake so Raven uses the distraction to attack and cripple Yang, intent on taking her back for more "training". Blake tries to defend Yang but clearly has no chance to win. Raven then offers Blake a choice; stay and die or run and live, believing she will run. Blake stays. Yang, not wanting her friend to die, begs her to leave. Blake stays. Raven then attacks and severely injures Blake. This doesn't sit well with Yang of course, who is able to drive her mother back in a rage fuled attack. Raven decides she's had enough and that Yang is a lost cause so she just leaves, taking Adam with her, leaving Yang alone with an injured Blake.
With this ending we get a Yang/Blake reunion, a perfect Bumblebee set up (if you ship it), Blake's done runnuing away and while Yang still has some issues to work out, she no longer has to go it alone. She also might have run into the Spring maiden in her time with Raven too.
Man, that turned out a lot longer than I expected.