请英语高手帮忙分析一下句子的结构，语法

but they suddenly became parents of quintuplets，four girls and a boy，an event which radically changed their lives.如果是：
four girls and a boy 是 quintuplets 的同位语起修饰作用，an event 做同位语 指前半句话，which radically changed their lives 是定语从句 修饰an event。那么句子主干是：they suddenly became parents of quintuplets，an event 怎么觉得这么别扭，虽然中文意思我懂？
答：这是新概念英语第三册第45课的一句话。
译：但他们突然成为了五胞胎的父母，四个女孩和一个男孩，（这是）完全改变了他们的生活的一个事件。
an event 前面省略了 which was

应该要去掉AN EVENT吧，主干里只能有一个主语

an event 指代前面的became parents of quintuplets，four girls and a boy，

这么用只是为了避免重复