The curly-spiral eyebrow is a Vinsmoke-family genetic marker. At the Whole Cake arc, Sanji’s siblings (Ichiji, Niji, Yonji, Reiju) are revealed with the same curly-eyebrow design — and the direction of the curl encodes which sibling is which. The Ch. 48 introduction is the earliest planting of the Vinsmoke-lineage genetic signature.
Predictive use: any other character later introduced with a curly-spiral eyebrow is a Vinsmoke. The design-element is Oda’s lineage-marker-as-visual-shorthand technique.
recent-chapter: the Vinsmoke family-name is not visible on-page in Ch. 48; the genetic-marker read requires far-future-chapter knowledge. Flag the eyebrow as seed in Ch. 48; the Vinsmoke identification is a forward-call.