His younger sister is studying economics.

他的妹妹在学经济学。

As a child she fought with her younger sister.

小时候她跟她的妹妹打过架。

She was furious at being upstaged by her younger sister.

她被妹妹抢去风头，感到气愤不已。

I gave my younger sister a pen.

我把一支钢笔给了我妹妹。

Is she your younger sister?

这个是你妹妹吗？