女 sits underneath as the indexing radical. 姿 originally focused on graceful feminine bearing — posture, comportment, the way someone holds herself. Modern usage applies to all genders (英姿, 雄姿), but the radical preserves the original framing.
次 supplies the sound, drifting from cì to zī (c-/z- is a regular alternation in old Chinese). The "order/next" meaning has no role here. Same phonetic appears in 资 zī (resource), 咨 zī (consult) — all sharing the -ī reading.
