Twoota tip #60: You will all die to a Sibling's hands. ~Oric
Sibling.
Oric is not a Sibling. For some reason, he doesn't want to kill us himself. This fits with the odd Let's You and Him Fight thing we've got going on here.
Simplest solution, we tell Oric where he can shove his Soul Star and see what happens.
Alternate solution (as I do think we'll be unable to trick him with ropes or magnet boots or windirection or etc, so -1 deception) would be to give Al the ring of Sexy and have him equip the Bartering Ram and attempt to negotiate with Oric (maybe talk him down to just maiming Riltia a little.) I think I'd rather just openly defy Oric at this point, though, since evidently ruining our relationship with Riltia is more important to him than us being dead.
If a non-illborn kills another non-illborn; they get Soul Charge. The Siblings are batteries. Riltia is a Sibling, Target is a sibling. We are, therefore, transferring charge into a single battery by killing Riltia.
And the inevitable death of Target ends the cycle. Emotions also cause Soul Charge gain. So either way, Ciro freaking out about this is what Oric wants, and no matter the outcome, he gets what he wants.
Saving Riltia via items likely wouldn't work, because Oric would probably be able to tell if she died or not.
He also, is not likely to prematurely end the cycle; but if he sees that we genuinely care about Riltia, he might deem that potential abnormality as a threat to the Cycles, and kill us. SOOOOOOOOOOO:
Refuse killing Riltia, make up the excuse of her gear and inventory space being essential to the party.
After Oric leaves, if he does, tell Riltia that you made up that excuse and that you are sorry for saying such things.