I think there are some connections between Aldebaran and the Oni twin sisters. That's my theory :
In the Arc 3, after Subaru was severely beaten by Priscilla, Al asked Rem (he named her Ram) to take care of Subaru. But Rem answered him she's not Ram but Rem and Ram is her sister's name. Then, Al questioned her if Ram was still alive. Rem approved (she thought he had already met her sister somewhere before).
After learning that, he gonna fucking mad. In my opinion, he was persuaded she's dead, killed by him more exactly. And also, he lost his left arm and his face was burned during his fight against her. However, there is only one possibility they can fight each other. That means before Roswaal hired both twin sisters or rather during the Oni village's attack.
And why ? Aldebaran was persuaded he killed Ram. But that made no sense if Ram survived and was severely wounded before or after this tragedy, or else Rem should learn it. Don't even talk about "during" !! In that case, she couldn't save her sister before or she couldn't use her full power face to him after losing her horn.
But why Rem can't remember what's happening next in her flashbacks ? Ram was unconscious and Rem was surrounded by a dozen of witch cult's members. There are two evidences I could say : Aldebaran was present in the village and Rem became insane (ONI MODE ACTIVATED !!!)
So Rem killed her last enemies, except Al, who is a great opponent. She cut his left arm by using El Huma and used a baton or something else with fire to hit several times his face to kill him. Then, Al stood up and came back to get his revenge on her. Thinking she was dead, he let her twin sister (Ram) alive and leave that place.
In the conclusion, Aldebaran could be the Archbishop of Pride. He attempted the Oni village and ordered his henchmen to kill both twin sisters. But after the clash between Al and Rem, in his mind Rem was killed by him (so he called her Ram and forgot Ram have pink hair) OR he thought Rem was Ram by judging her power (but he called her Ram because he forgot what exactly is the name of Rem). Now, that makes him sick Ram (more exactly, Rem) is still alive, because he failed to kill her target.
Well, how do you think about my theory ? Please, give me your own opinion !