Perhaps it is because I am the youngest child and the only sister they have.

也许是因为我是最小的孩子，也是他们唯一的妹妹。

youdao

If the only sister of your mother's only brother has an only child, what is that child to you?

如果你妈妈唯一的弟兄的唯一的姐妹有个唯一的孩子，这个孩子是你什么人？

youdao

My sister is the same height but only a size 12.

我妹妹的身高一样，但只穿12码的。

《柯林斯英汉双解大词典》

Her only hope is a baby sister who was genetically engineered to help save the older girl's life.

VOA: special.2009.06.12

Her neighbors seemed largely sympathetic, noting that Bernardo's only sister died young and that she never knew her father.

CNN: Teen offers virginity for money

In my photograph, for example, I see only my sister, as does she.

BBC: Seeing double

Prior to the film, Hemingway only saw her sister Joan Hemingway about once a year.

CNN: Hemingway family mental illness explored in new film