Question

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？

Answer 1

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

Answer 2

应该要去掉AN EVENT吧，主干里只能有一个主语

Answer 3

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