What we have now to see is if Sanji's attack power while doning the RS is on par with his durability. If he can damage King as easily as he tanked his attack, then he's now a YC1 level monster, otherwise he should be YC 2-3, since no matter how much durability the RS provides, he could end up in the end like Ichiji did against Katakuri.
About the siblings, I'm on the fence about who's stronger, to be fair.
About Niji, Sanji did dislocate Niji's jaw with a kick, but Niji didn't seem like he's a weakling without his RS either, and how his electricity power actually comes from his body it's possible that RS-Niji is actually much stronger than he had the chance to demonstrate. I think Sanji with a RS is stronger than Niji, unless Niji really had much else to show.
About Ichiji, I believe Ichiji is stronger than Sanji, RS or not, until Sanji has a feat that proves otherwise. I don't know exactly how strong or weak Ichiji's basic CQC is, since he really didn't show anything, but Ichiji pierced through Oven's body with his special ability.
Oven was praised as having the strongest durability out of all of the Charlotte sons. He ignored kicks from both Sanji and Niji, he was run over by a ship and hit on the head with a giant club, and he ignored all of it, but Ichiji pierced his shoulder all the way through with a single skill, so Ichiji is definitely a force to be recokned with despite how Katakuri humiliated him. His attacks destructibility are definitely top tier, but he lacked good CoO to beat Kata.
If Sanji proves to be YC-1 level all around, then I'll accept that he has definitely surpassed Ichiji as a fighter.