This is a tintype of a woman whom I believed to be Sarah Lewis "Sally" Hughes McComb (1812-1892), wife of William McComb. I had no proof of this, but was basing this on the resemblance between this woman and Elizabeth Jane McComb Witt, who was the daughter of Sarah McComb.
As Elizabeth's chin seems to be broader, her eyes seem to be set a little wider apart, and her mouth is not as wide and is more bowed than these features on the woman on the left, I didn't really consider that both of these may be pictures of Elizabeth.
Tintypes didn't become popular (from what I've been able to find out) until about 1850, and Sarah L. Hughes McComb was born in 1812. While the unknown woman appears to be older than Elizabeth, I don't know if she is old enough to be Sarah.
Some people who've looked at these pictures think that they are of the same woman--a woman who in 1859 was probably well taken care of, and who then, just a few years later, was caught up in the stress and hardship of the Civil War.
Thus far, attempts to measure the faces to attain ratios haven't been completely conclusive, though there do seem to be significant differences, including the ones that I pointed out, above.
So I don't know. Are these pictures of a mother and daughter, or are they pictures of Elizabeth, taken before and then during the War?