我有一个弟弟。

I have a younger brother.

《新英汉大辞典》

我有一个姐姐、一个妹妹、一个弟弟。

I have one older sister, one younger sister and one younger brother.

youdao

该新确诊病例为该最初病例的一个弟弟。

The newly confirmed case is a brother of the initial case.

youdao

According to the Second Brother, it's virtually impossible for a single, helpless maiden to pass uninjured in this wild, surrounding waste.

弟弟认为，一个孤独无助的女孩想要，毫发无伤的穿过这野外的荒原是根本不可能的。

耶鲁公开课 - 弥尔顿课程节选

Because her brothers have naturally done what any brother would do accompanying a sister in a dangerous forest at night.

因为她的弟弟,做了任何一个弟弟深夜跟随着姐姐,在一个危险森林会做的事。

耶鲁公开课 - 弥尔顿课程节选