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?