Among them will be Debbie's mother,who swam the Channel herself when she was a girl.语法请教

Among them前置的表语// will be系动词// Debbie's mother主语,who swam the Channel herself when she was a girl. 逗号后面是非限制性定语从句，修饰主语。
这里herself其实是反身代词最常用的一种用法， 用作同位语(加强被修饰词的语气，紧放在被修饰名词后, 或句末)：
The box itself is not so heavy. 箱子本身并不重。
Don’t trouble to come over yourself. 你不必费神亲自来了。
这句里的Herself也是可以移到who后面的。