The girl to whom I spoke is my cousin.

The girl to whom I spoke 是主语，is 系动词作谓语，my cousin 作宾语。
主语中to whom I spoke 是从句，修饰the girl。
在从语中，可以理解为whom I spoke to，whom是宾格，spoke to 表示对谁说话，此处是将介词to提前。

是对的，也可以说the girl whom i spoke to is my cousin.
主语the girl
从句to whom i spoke to

是定语从句。
The girl (to whom I spoke) is my cousin.
括号里面的部分就是从句。可拿掉语义不变。

也可写成whom I spoke to

也可以说成

the girl who i spoke to is my cousin.