They are, from what I can gather they go to school together but are enemies on the battlefield. Sort of Sailor Moon-ish I suppose. You can see them more here or on dannychoo.com if you want still shots.
I spy a Miku cameo. Makes ya wonder where the Twin Tails on Black Rock come from when not in school I expect Dead Masters horns burst out of her head. Unless it's already shown how that works out, I haven't looked into the anime much.
Actually the anime bases on the story which is still is the older sister turning someday because of evil spirits to Death Master which is also used at the 3D pv...
"A little old, two sisters just had spent life on the streets of the idyllic countryside.
They flee, while supporting each other, passed the parent happily. One day, the younger sister did run up to the older sister, the stomach of the younger sister was penetrated with the enormous sword of the older sister.
The younger sister don't recognized the reason, deteriorates and falls.
Black wings and horns grew from the older sister in front of wierd skulls, the older sister disappeared infront of the younger sister.
The sister survived by a miracle regaining consciousness, the town wakes up to her and her people had been thoroughly destroyed by the massacre. The older sister and her, were the persons who said Dead Master needs to be destroyed, the Bone Legion soldiers will be notified, the emergence and transformation of her at the same time.
Dead Master leading the invincible bone soldier corps, in order to ruin all mankind, had started conduct. The bone soldier intelligence is low, their attacks are just like those from dogs but struggle not only against simple attacks. They reject any attack on their side. They also avoided the specter of weapons and bullets to exist as a substantive. The army can be defeat, the only bone that is decorated with weapons attacks by a special processing...
The people just had few weapons, but didn't handle. BRS is an ancient weapon that can be made against bone soldiers which would help the yonger sister. Rock Shooter and Black Sword were received and the older sister stops, the variable motorcycle, becomes the moonlight circle and also appears traveling as a Black Rock Shooter..
Then a few months later, she awakened from a nightmare and is lying in ruins standing in the middle of the journey. "
But that doesn't seem like the base for BRS Animated. That -to me- is a fan who wanted to try and do a BRS story himself. Since a lot of people speculated that they were sisters.
I heard the story is just like what Ayu wrote and huke, the artist and author of BRS, requested it just like that himself. Remember that huke created BRS a long time ago even before the BRS song sung by miku.
I am aware that the Huke's art inspired ryo. ^_^;; I wrote the wiki for it.
I am referring to the story that is coming up for BRS animated, in comparison to the fan video. When it came to the mecha robot (what looks like one), dragon knight, and combat maid; it just doesn't look very polished. So I think things will change for the release.