E.

This article made incorrect assumptions. I understand that you may not know other materials that have been released, but even the quoted text itself rebuts the article:

"I had already planned to have Sanji have siblings for quite a while, but my daughter loved an anime called Osomatsu-san with sextuplets just before the Whole Cake Island arc".

Please read that sentence more clearly. He had planned more siblings even before his daughter became a fan of Osomatsu.

I previously wrote a comment saying that Ichiji, Niji, and Yonji has been planned for a long time. That right there proves my claim. In fact, in one magazine, it was revealed that originally there is a fifth brother, named Goji. Eventually Oda replaced Goji with Reiju.

The only thing that Oda changed was their ages.

Previously, Ichiji and Niji were designated to be a few years older than Sanji, with Yoji and Goji a few years younger. However, Osomatsu made Oda change it so that Sanji and his brothers were quadruplets of the same age.

I don't know what happened to my previous comment, maybe moderators deleted it. Anyway, my critique/correction is made with proper arguments backing me up.

The article links to/made reference to an older article about how Sanji was originally meant to just have one sibling, but this is false. The article's writer did not provide an interview quote to back up this claim, so it could be just a fake rumour in fandom communitiies or personal opinion.

Here I show a proof of an old draft Oda made that was released in a magazine. As I said, there were five brothers and no Reiju. Reiju is made later. The designs of the character appearance also have not been finalized.