我来简化你的理解.
用括号把to Meg from her mother括起来,并且把它放到句首不管.这样句子就简单了--------
（to Meg from her mother）Tina brought a message SAYING that hat all was well at home and she needn't worry
a message SAYING (这是分词做定语)=== a message that said （这是句子做定语）
saying放在message的后面是因为saying后面自带了一个句子 that hat all was well at home and she neesn't worry.
现在,你明白了吗?