大dà义yì灭miè亲qīn

Literal: for great righteousness, destroy one's kin
Figurative: To put justice above family ties, even turning in a guilty relative; jokingly, it is also said of someone who tells on a family member.

Where does 大义灭亲 come from?

It comes from the 《左传》. In the state of Wei, Prince Zhouxu killed his brother, Duke Huan, and seized power; his chief accomplice was Shi Hou, son of the minister Shi Que. Since the usurper could not win over the people, Shi Que advised them to seek the favor of the Zhou king through the lord of Chen, and secretly asked Chen to detain them. Wei had Zhouxu put to death, and Shi Que sent a man of his own to kill his son. The chronicle praises him as a minister of integrity and applies to him the formula "for great righteousness, destroy one's kin." Today it is used both seriously and as a joke.

Source (出处): 《左传·隐公四年》

How is 大义灭亲 used? Two examples

发fā现xiàn弟dì弟di在zài帮bāng人rén洗xǐ钱qián后hòu，他tā大dà义yì灭miè亲qīn，直zhí接jiē报bào了le警jǐng。
When he found out his brother was helping launder money, he put the law before family and went straight to the police.
女nǚ儿ér大dà义yì灭miè亲qīn，把bǎ爸bà爸ba偷tōu偷tōu抽chōu烟yān的de事shì告gào诉su了le妈mā妈ma。
Their daughter sold her dad out without a second thought and told her mom he'd been sneaking cigarettes.

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