GMAT Sentence Correction (SC) | Re: Emily Dickinsons letters to Susan Huntington Dickinson were

As the answer is option E, the correct sentence turns out to be :

Emily Dickinson’s letters to Susan Huntington Dickinson, which were written over a period beginning a few years before Susan’s marriage to Emily’s brother and ending shortly before Emily’s death in 1886,outnumber her letters to anyone else .

My question is :

In the phrase 'outnumber her letters to anyone else', what does the pronoun 'her' refer to ?

As per the meaning, 'her' seems to refer to Emily Dickinson. Can it