Sarah is alleged to be a daughter of William KELLEY & Eleanor CORBIN, but I have not found proof.

— born in Huntingdon Co., PA:
  9.  Benjamin CORBIN, b. 19 Feb 1807 
10.  Elijah CORBIN, b. 1804-10; d. age 20

Most secondary sources have all the children born in Huntingdon Co., PA, but the 1850 Censuses (and other records) show that all, except Benjamin, were born in Maryland.
If Edith really was born in 1785, then she is either Caleb's twin or Sarah was full-term pregnant when she married Elijah — or Edith is not their daughter.

1a.  D.L. Murray & N.R. Murray, compilers.  1995. Marriage Index: MD, NC, VA, 1624-1915 (Broderbund CD-4):

Corbin, Elijah

Kelly, Sarah

Feb 16, 1785

Baltimore Co., MD

1b.  Marriage Index:  Maryland, 1655-1850.  Liahona Research, Orem, UT (Broderbund CD-224):

Corbin, Elijah

Kelly, Sarah

16 Feb 1785

Baltimore Co.

1c.  Robert Barnes.  1978/93.  Maryland Marriages, 1778-1800.  Genealogical Publ. Co., Baltimore, MD (Broderbund CD-195).

p. xii	BALTIMORE COUNTY... 51 BA - Marriage Licenses issued by William Gibson, Clerk of Baltimore County, between 15 Dec. 1784 and 24 Nov. 1787.  The minister who performed the ceremony is in parentheses. Copied from Liesenring, "Maryland Marriages, 1777-1804," at MHS [Maryland Historical Society].
p. 45	Corbin, Elijah, 16 Feb. 1785, by Rev. Davis; Sarah Kelly 51 BA-29

2.  Letter, dated 19 Jun 1858, from Keziah (GREENLAND) SHANAFELT (click her 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, and Lorinda Clendenon Greenland generously posted a transcription at her web site. 

...Uncle Elijah and Mycajah married sisters, Sarah and Mary Kelly.  They both died in Cassville since I came to Clarion....

3.  Census Index:  U.S. Selected Counties, 1790 (Broderbund CD-311):  can't find.

4.  Census Index:  U.S. Selected Counties, 1800 (Broderbund CD-312):  there was no Federal Census in 1800 in Baltimore County at large, only the City of Baltimore was enumerated in 1800.

5.  Census Index:  U.S. Selected Counties, 1810 (Broderbund CD-313):  can't find.

6.  1820 Census Index and Digital Images (online at Ancestry.com; Image #3 of 6 — indexed "Corbeil"; extracted by Diana Gale Matthiesen):

1820

PA

Huntingdon Co.

Union Twp.

p. 42

Elijah Corbin

020001-00101-0-100

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	10-15	1804-1810	= Benjamin (b. 1807) = Daniel
1 male	45 or over	in or bef. 1775	= Elijah (b. 1760/4)
1 female	16-25	1794-1804	= Iantha? (b. 1794)
1 female	45 or over	in or bef. 1775	= Sarah (b. 1763)
1	person engaged in agriculture

Also living in Union Twp.: George, William, and John G. CORBIN.

7.  1830 Census Index and Digital Images (online at Ancestry.com; Image #5 of 18; extracted by Diana Gale Matthiesen):

1830

PA

Huntingdon Co.

Union Twp.

p. 16

Elijah Corban

010 000 001 - 000 001 100

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	5-9	1820-1825	= grandson? step-son?
1 male	60-69	1760-1770	= Elijah (b. 1760/4)
1 female	30-39	1790-1800	= daughter? or Elizabeth (b. 1790)
1 female	40-49	1780-1790	= Elizabeth (b. 1790) or daughter?

Listed next to Samuel GREEN, who is listed five lines from John CHILCOTE, who is five lines from Zacharia LOVEALL; and, in the other direction, two lines from Henry ELEY, who is six lines from Elijah HOUCK.  Also living in Union Twp.: Asa CORBIN and Benjamin CORBIN.

8.  1840 Census Index and Digital Images (online at Ancestry.com):  not found, as expected.

9.  Pennsylvania Will Testators Indexes [link died] (online at SAMPUBCO).

Testator	Residence	County-Vol-Page
CORBIN, ELIJAH	UNION	31-4-66
31 = Huntingdon Co.; Vol 4 dates from 1835 to 1847

10.  Irene A. Nielsen.  1983.  Query.  Maryland Genealogical Society Bulletin 24(2): 175.

CORBIN, LYE, KELLEY.  Parents 3 MD CORBIN bros.? (1) Benjamin d. ca. 1815, m. Sarah LYE 9 Dec. 1755, St. John's Parish, Balto. Her parents? (2) Abram bur. nr. Benjamin family farm Westminster, MD. (3) William d. ca. 1822 Huntingdon Co., PA.  Elijah CORBIN, son Benjamin, m. Sarah KELLEY 16 Feb. 1785, Baltimore Co. Her parents?

11.  LDS.  Family Search: Internet Genealogy Service: Ancestral File.  Says Elijah was born in 1760.  The 1760 date seems more likely than 1764 because, otherwise, he was younger than his wife, which would have been very unusual.

12.  Broderbund.  World Family Trees.  Vol. 40, Pedigree No. 356.  Says Elijah was born in 1764.