It’s difficult to tell the age of this subject, as she looks mature, young, matronly and youthful all at the same time. The hair bow on the top of her head makes her look like a child in some ways and it’s not a flattering look. Her wide collar reminds me of the types of collars early teens wore in the 19th century, but at the same time her dress follows all the lines and style of a woman’s dress. She is an enigma, I’d guess about age 20. This is the top left of the second open page of the Dobb Long Book.
Speaking of the title of the book, when I was looking at it tonight I noticed it very specifically says Dobb “Long Book” suggesting that it isn’t two families, as Iggy speculated. I don’t know how I missed this when I introduced the book to you.
The photographer here was Hartley at 309 West Madison St, Chicago. A later example of the back of a cabinet card by Hartley can be found here showing “three immense floors alive with the best photographic talent possible to procure!” I believe this image in our collection to be from the early 1880s.
Who Were They? 
I would agree with your age guess – she appears to be wearing a wedding ring – so she may have been a young(ish) bride that grew up quickly which would give her that “mature, young, matronly and youthful” look.
Do you think this might be Nina? If so, and if she is newly wed, I would speculate some more – and say her marriage in around 188X would explain why she’s “invisible” to census records – she would be listed under her maiden name in the 1880 one and she died just before the 1900 one.
Thanks for clearing up the “Long Book” – I’d have to agree that it refers to the long and skinny photo album.
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18 maybe 20 ..and I think it is her hair style that makes her look childlike. I thought the buttons on her sleeves were interesting..you don’t see a sleeve with two kinds of lace very often and then buttons too. :)
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