Sarah Bell Douglass had a sister named Dora, probably named after Benjamin's mother Dorothy Kench. She was born in 1841 in Canada. Dora married Charles H Graham on 27 Dec 1860 in Jackson County, WI. There was one other child in the various census records, a Rozetta/Rosetta born 1850 WI. Not sure if or who she marrried. I haven't seen evidence of any other children of this marriage, but it was very early times in WI, and Aztalan was quite remote.
I don't know if this helps answer your question, but Benjamin Douglass did have a sister Louisa, born 1818. The website Wisconsin History shows that a Louisa Douglass married Samuel Belding in Jefferson County WI on 6 Feb 1842. I haven't been able to find any trace of her beyond this. I believe this Samuel Belding was the son of Aaron Belding and Leonora Powell. Aaron died not long after Samuel was born, and Leonora remarried to Silas Wheeler. If I am correct, Louisa had 4 children with Samuel: Adelia (1844), Henry (1849), Benjamin Franklin (1852), and Aaron (1854). She appears to have died before 1856, when Samuel married Altheda Chandler in Fillmore Co, MN.
Best of luck in your search for Louisa!