Perhaps Oric's Soul Charge just charges absurdly slow and he needs to do this to get power. His speech about the prison makes me think that maybe he is trying to recreate the prison, but not for his own benefit. Like maybe the siblings can no longer survive in the outside world and the cycle and the Warrens is to keep them in check. Sort of like they became dependent on the prison and not being in the prison causes them to freak out... overloading their Soul Stars and... I think I going to far of the deep end here... so I'll just stop.
I'm not sure that that is too far off the deep end, though there are a few obvious flaws with the theory they aren't unsolvable.
For instance, if Oric's Soul Charge is so slow, why is he the one that stays alive, as opposed to one of the others?
Well, it might be that he was just the last one to not die and recycle, whether that be due to the Kingdom or Cain or whatever. If all of the others were killed and reborn different, they wouldn't have the same goals, and Oric might not be able to rely on them.
Alternatively, maybe his Soul Charge isn't all that slow, but it just takes a gigantic amount of power to do whatever is needed (plus possibly keep Cain bound? And/or keep the dungeon running?). Remember, when we went into Oversoul state it didn't last for very long at all, and if there are a lot of things he has to do, well...
Also, given what it took to restore Riltia from, er, FUN mode, and my speculation about the recycling of the siblings changing them too much, he may be planning to gather enough power to return them back to how they were before, on top of whatever else he wants to do. Perhaps the Kingdom set a similar system up, and Oric wants to undo whatever they did?
I suspect that bit about Cain killing siblings is important for more than the suggestion that they provide more Soul Charge.