句子可以写成以下形式便于理解和翻译
It was customary that the father was boss in the traditional family.
在传统的家庭里,父亲当家做主是惯常的事.
回到原句
the traditional family,in which the father was boss,was customary.
要先翻定语从句,即 in which the father was boss,which指代 the traditional family.父亲在传统的家庭里是当家作主的.做这个句子的主语.
然后再翻译 谓语 表语 was cutomary.表示 (父亲在传统的家庭里是当家作主的) 这件事,是惯常的.