姊

Left-side woman radical - pictograph of a kneeling figure with crossed arms. The indexing radical here, supplying the meaning directly: 姊 names an older sister, and like 姐, 妹, 妈, 嫂 it sits squarely in the family-relations cluster built on 女. The radical's pictographic posture conveys the quiet weight of seniority within the household.

Right side is the contracted graphic residue of phonetic 朿 - cì drifting to zǐ through an old initial alternation. Standalone 朿 is 6 strokes; here it compresses to 4, so the historical pieces have visually fused into a thorn-like vertical shape. 朿 pictured a barbed branch; the meaning of older sister rides entirely on the woman radical.