The Elizabeth SIAS in the 1790 Census of Patapsco Lower Hundred, Baltimore Co., MD, has been presumed to be John's widow, but I've yet to see any proof that she is, and her name argues against it.

1.  Sarah SIAS, b. 1736-40, Baltimore Co., MD; m. Benjamin CORBIN
2.  Joseph SIAS
3.  Benjamin SIAS (Sr.)
4.  Keziah SIAS, b. ca. 1761?; m. Samuel LANE 

There is a James SIAS (b. ca. 1755-60) in the 1790 census of Baltimore Co., MD, and later censuses of Huntingdon Co., PA, who is almost certainly related to this family, and he is the right age to be a sibling of the above.  He is not mentioned by Keziah (GREENLAND) SHANAFELT in her 1858 letter, though her knowledge of her grandmother's siblings may not have been complete.  Note that James SIAS shows up listed just two lines from Keziah (SIAS) LANE in the 1800 census of Huntingdon Co., PA. 

1.  Robert Barnes.  1975.  Maryland Marriages, 1634-1778.  Genealogical Publ. Co., Baltimore, MD (Broderbund CD-195).

p. 163	Siahs, John 21 June 1736, Mary Chambers	3 BA-155
p. xi	BALTIMORE COUNTY... 3 BA - St. Paul's Parish; copy made by Harrison, at MHS.

2.  Letter, dated 19 Jun 1858, from Keziah (GREENLAND) SHANAFELT (click Keziah's link for a full transcription) of Clarion Co., PA, to her son, Thomas.  Keziah is a granddaughter of Benjamin & Sarah (SIAS) CORBIN.  The letter is in the possession of  Laurel Shanafelt Powell of Colorado; Lorinda Clendenon Greenland generously posted a transcription at her web site. 

My mother's father was Benjamin Corbin; her mother Sarah Sias.  I think they both came from England.  Their ancestors lived in England where some of them still remain...  Grandmother had two brothers -- Joseph and Benjamin Sias.  I remember seeing them many years ago.  They moved to Ohio.  She had a sister named Keziah Sias...

3.  1790 Census Images (online at Ancestry.com):

1790

MD

Baltimore Co.

Patapsco Lower Hundred

p. 204

Elizabeth Sias

0-0-2-0-0

These data indicate:

Gender and Type	Age Class	Therefore Born	Individuals Inferred
2 free white females	all ages	in or bef. 1790	= Elizabeth = ?

Elizabeth has been presumed to be John's widow, but she could just as easily be someone else's widow or even John & Mary's unmarried daughter.  Daughters Sarah and Keziah were married by 1790, so the second female is neither of them.

4.  Broderbund.  World Family Tree.  Vol. 1, Pedigree #2346.  Has John's wife as Elizabeth CHAMBERS.