Clara Harris Rathbone photo mistaken again

[GARY POPOLO]

I have a question some one may be able to answer. I am reading Lincoln's Boys and the book was explaining why Rathbone ended up becoming Lincolns theater guest along with his STEP-SISTER Clara Harris who he would later marry. I never read that she was his step-sister. Can anyone clear that up for me? Maybe I missed this piece of history along the way.....