His younger sister is studying economics.

他的妹妹在学经济学。

《柯林斯英汉双解大词典》

As a child she fought with her younger sister.

小时候她跟她的妹妹打过架。

《柯林斯英汉双解大词典》

She was furious at being upstaged by her younger sister.

她被妹妹抢去风头，感到气愤不已。

《牛津词典》

He had a younger brother, Abbott Lawrence, and a younger sister,Amy.

VOA: special.2010.07.11

You remember they're wandering around the forest without her picking berries. The younger brother, - the second brother, is most concerned about his sister's safety -- so I want you to look at this passage.

你们记得他们在树林中游荡,没有拿他们采摘的果子，小哥哥，也就是二哥哥，是最关心他妹妹安全的-,所以我想让你们看这篇文章。

耶鲁公开课 - 弥尔顿课程节选

Let's say that you're talking to your younger brother and sister.

假设你正在和弟弟妹妹说话。

It's important 课堂 - SpeakingMax英语口语达人

Friends describe Ms. Stunt, the younger sister, as the shyer of the two, but more free-spirited.

WSJ: The Ecclestones: The New First Family of Real Estate

LaBrot works side by side with his younger sister, Sky, to run the all-volunteer organization.

CNN: Bringing health care to the world's most remote areas

She described an unhappy childhood with little affection given to her or her younger sister Freda.

BBC: Coronation Street actress Betty Driver dies, aged 91