William D. Elliott was married to Joanna Boyd in 1847, according to the 1900 census records. William's birthday is listed as Feb 1825 and Joanna's is Jan 1827. Eliza may have been another name Joanna had or perhaps he married Eliza before Joanna. Either way, Joanna is listed in every census from 1850 through to 1900 with William and their children. Those children are, as you have stated, by age:
Marion/Marian A. Lauretta "Laura" J. Joanna Elizabeth "Lizzie" Emma S. Ada Jeanette Mary Cora Sarah "Sadie" Boyd J. Jessie Maude Anna "Annie" William
The family lived in Hopewell, Beaver, Pennsylvania in 1850, traveling to settle in Hookstown by 1870 and Pittsburgh by 1900. Both of his parents are listed as being born in Ireland. Joanne's father was Pennsylvanian and her mother is listed as being Scottish. The census played around a lot with the spellings of their first names and tended to use nicknames, which is where a lot of the "Lizzie," "Annie" and "Sadie" types come from.
I am tracking my own Irish Elliott family and have run into this line via DNA results and locality (my Elliotts are also from Hookstown) but so far have not found any paper trail proving a connection or lack thereof. I'm just posting this information in the hopes of helping someone else.