手足 ("hand and foot") metaphorises siblings as parts of one body. The idiom is grounded in Tang poet Li Hua's 《
吊古战场文》 ("Lament for the Ancient Battlefield"):
谁无兄弟，
如足如手 - "who has no brothers, like foot, like hand?" - and reinforced by Song writer Su Zhe's memorial pleading for his imprisoned brother Su Shi.
