I met two girls,one of whom is my cousin

逗号嘛,简单来说就是表示定语（定语就是修饰宾语的话）的作用了,如果你在逗号后面加个and或者but,就不是定语了,是另一句话.就要用one of them is my cousin.但是这里有个逗号,后面没有连词,就表示这是个定语,就要用one of whom is my cousin.你可以记住,一般都是这种题型,I met two girls,one of whom is my cousin.或者I met two girls,and one of them is my cousin.