In my 3/29 post I mentioned that I was trying to figure out who Martha Demaris was that appears in the 1900 Census with Frank and Catharine Smith, and listed as sister-in-law. Mom comes through again. She found a Jacob B. Demaris in the General Index to Pension Files, 1861-1934, in which a Martha A. Demaris sought a widow's pension for his service in the K 59 Ohio Infantry after Jacob's death in 1916.
So I asked Mom, why sister Martha might have been living with the Smith's in 1900. She posited that since Catharine had lost a leg, perhaps Martha had come to stay and help during her convalescence. We don't really know when she lost her leg, but know it was later in her life. A life crisis like that could account for an extended stay that would have put her there during the census.