brothers and sisters are at home.
at home是在家的意思。既然在家几不存在在他们的家。‘

我们说在某人的家，这样说： My brothers and sisters are at their grandparents'place.

Hope my explanation is correct.

Mr Brown's family ____having lunch at home.应用are
The brother and sister are at their home.改为The brother and sister are at home.是不是合适一点.

你们好啊

第一个用are 第二个把their去掉

字典上有原句,in their homes