Where does “兄弟分” come from?

兄弟分 (Japanese) comes from Japanese 兄弟, from Japanese 兄, from Old Japanese あに, from Japanese の, from Old Japanese の, from Middle Chinese 能 — talent, ability.

兄弟分 (Japanese): sworn brothers

Definitions

sworn brothers

Ancestry of “兄弟分”, step by step

兄弟分 traces back through two separate lines of ancestry.

via Japanese 兄弟

Step	Language	Word	Meaning
1	Japanese	兄弟	brothers; siblings; brothers and sisters;...
2	Japanese	兄	elder brother; elder brother-in-law; Short for...
3	Old Japanese	あに	—
4	Japanese	の	genitive case marker; nominative case marker in a...
5	Old Japanese	の	plain, field
6	Middle Chinese	能	talent, ability

via Japanese 分

Step	Language	Word	Meaning
1	Japanese	分	a minute; a part, a share, a portion; an obsolete...
2	Middle Chinese	ふん	—
3	Middle Chinese	糞	—

Every word from Middle Chinese 能 →Every word from Old Japanese の →Every word from Japanese の →