Liǎng jiā gōngsī xiōngdìxìyúqiáng, jiéguǒ bèi duìshǒu qiǎng le shìchǎng.
The two companies squabbled like brothers at home, and a competitor grabbed the market.
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From the 诗经 (小雅 · 常棣): 兄弟阋于墙, 外御其侮 (brothers may quarrel within the walls, but outside they fend off insults together). The shortened form 兄弟阋墙 also exists, meaning 'internecine strife.'
usage
阋 (xì) = to quarrel, an uncommon character that is easy to misread. 于 = at / within. Often paired with 外御其侮.
