Of course, there's the very good chance that this won't be enough Soul Charge and will just result in possibly killing a major party member for no gain, hence why I say we keep it as a backup plan rather than an instant one.
That image shows the amount of soul charge we had before Al died and we went oversoul. The amount we have now is well over the amount we had back then, plus, we don't even LIKE Al as much as we like Riltia, so the soul charge we might get if she were to die would be even greater.
Perhaps Oric knows this, and is the reason why he is hesitant to kill Riltia himself. Oric sees Ciro and Riltia's alliance as a threat, but he could be afraid that killing Riltia would cause Ciro to get incredibly pissed at him and go oversoul. So, he wants to force Ciro to kill Rilitia instead of doing it himself as that might weaken Ciro's soul instead of charge it.
EDIT: Now that I think about it, Oric want's Ciro's soul to charge, so he could be wanting Ciro to kill Riltia simply because it would charge his soul. However, I wonder about whether or not Oric would be happy if Ciro went oversoul. Perhaps Ciro going oversoul would provide him with more soul charge, but from what we saw before, Ciro's charge was empty by the time oversoul ran out, so maybe Oric wouldn't want that.
Another thought, since Oric is trying to harvest as much soul charge from Ciro as possible, the reason he doesn't want us to ally with the other siblings is probably because the other siblings are either meant to kill Ciro or be killed and charge Ciro's soul. Kind of sick when you think about it, using your siblings as a farm to harvest energy...
... I think this has probably been noted before but if it was, I forgot, haha