This idiom comes from a old folk song. The last two stanzas go like this:

One family has five brothers, all serving to a minister as attendants. Every five days, they go back home for a reunion, decorating their horses and garments with shining gold. They vie with each other for ostentation and extravagance, attracting crowds of onlookers along the road.

Now there is a peach tree by a well, and a plum tree next to it. When worms come to gnaw at the root of the peach tree, the plum tree invites them to gnaw at its own root. Finally, the plum dies, ossified.

Even trees know how to sacrifice for other trees, why can't brothers do the same?

这个成语来自于一首古老的民歌。 歌曲的最后两段说的是：

一户人家有兄弟五人，都在一个大官的家中做仆人。每隔五天，他们会回家团聚一次。他们用金子装饰马匹和衣服，吸引了许多路人来看。五人互不相让，相互比较谁的排场大。

如今一口井边有一棵桃树，桃树旁是一棵李树。有虫子来咬桃树的根，李树便让虫子来咬自己的根。最终，李树变得僵硬，枯死了。
连树都知道为了别的树牺牲自己，为什么兄弟之间反而做不到呢？

李代桃僵 (lǐ dài táo jiāng)
【翻译】The Plum Tree Sacrifices Itself For The Peach Tree-Sacrifice Oneself For Another
            Person.
【释义】僵：枯死。李树代替桃树而死。原比喻兄弟互相爱护互相帮助。后转用来比喻互相顶替或            代人受过。
【例句】那个人不是杀人凶手，却被冤枉入了监狱，真是李代桃僵啊！
【近义词】代人受过