We also got the knowledge that after a specific number of cycles siblings are forced to resurrect after regardless of if the target kills them. This could be something built into the Warrens, but Cain doesn't seem to have resurrected in quite some time, possibly never. So I find it more likely that it is something Oric forces (and he is too afraid of Cain to force it on Cain.
Now then, why? To control the siblings? Quite possible, by wiping their memories periodically he keeps a monopoly on knowledge, and knowledge is power. To harvest Soul Charge? I would say yes. The process of resurrection could harvest Soul Charge from the siblings, as well as increase the soul charge of other siblings who just say someone they knew (well) lose all of their memories.
I would guess that the siblings of the earlier levels are wiped more often. Riltia is probably wiped every cycle and knows nothing, Proxxy is wiped not every cycle but often enough that she doesnMt form strong connections or gain much knowledge, Cain is never wiped because he scares Oric (or at least is a threat) and so has a dangerous stockpile of knowledge, but Oric relies on the fact that the other siblings are scared of Cain and don't work with Cain very much so Cain doesn't spread too much knowledge, except Ecco and Cain are currently working together and Oric is NOT happy about that. Continuing, Ecco and up have enough time to gain rebellious thoughts, but doesn't worry Oric until Oric realizes he's working with Cain.