The two sisters will look after their baby brother when their mother isn't at home.

妈妈不在家时，这两姐妹将照看她们的小弟弟。

youdao

I can remember the precise moment when my daughter came to see me and her new baby brother in the hospital.

我还记得我女儿来医院看望我和她刚出生的弟弟的那确切的一刻。

《柯林斯英汉双解大词典》

You've got a baby brother and a baby sister.

你有一个小弟弟和一个小妹妹。

youdao

Palin says her 7-year-old daughter loves showing off her baby brother to state officials.

NPR: Gov. Palin Has Reputation For Independence

My baby brother, Sergeant Mark Nagl, has been in for three years and was back on mid-tour leave in July.

NPR: U.S. Turns More Attention to Training Iraqi Forces

Aged 18, Linda said it was the best day of her life as she had always wanted a baby brother.

BBC: Pregnant UK servicewoman: Sympathy for plight