Well, I haven't solved the mystery entirely, but I found a few clues. When Amy Stewart, age 15 married William Milliken, age 18, there is a notation that the marriage was with permission of the Probate Court. This would lead me to believe that one or both of Amy's parents were already deceased at the time of Amy's marriage. (attached)
Actually, Amy is listed as Annie Stewart on the Lawrence, MA birth register for 1892. Her father is listed as a fireman.
James A. Stewart is listed as an engineer when he married Martha Dennison.
James A. Stewart, fireman, is listed as living at 3 Melrose place in the 1898 and 1899 Lawrence, MA city directories.
But he does not appear in the 1900 Lawrence city directory.
There is, however, a Mrs. Martha Stewart in the 1900 Lawrence City Directory, boarding at 445 Canal. It doesn't mention that she is a widow. Strangely, I don't find Martha Stewart on the 1900 census.
I'll see what else I can find.