Brother certainly:  William CORBIN
Sister almost certainly:  Sophia (CORBIN) MARSH
Brother very probably:  Abraham CORBIN

Some secondary sources give Sarah these parents, 
Father said to be:  John SIAS
Mother said to be:  Mary CHAMBERS
but I have found no proof of the connection.

If Asahel really was born in 1774, then he has to be the twin of Sarah Ruth.

1a.  Henry C. Peden, Jr.  1987.  St. John's and St. George's Parish Registers, Baltimore & Harford County, Maryland, 1969-1851: transcribed from Microfilm in the Maryland Historical Society Library in Baltimore, Maryland.  Family Line Publ., Westminster, MD (Broderbund CD-178).  On p. 99:

Page 212

Benjamin Corbin and Sarah Sye married December 9, 1755.

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

p. 40	Corbin, Benj., 9 Dec. 1755, Sarah Sye (or Lye)	2 BA-212
p. viii	2 BA - St. John's and St. George's Parish; copy made by Harrison, at MHS [Maryland Historical Society].  (Despite the title of the manuscript, the records are chiefly those of St. John's Parish.)

1c.  Robert W. Barnes.  1989.  Baltimore County Families, 1659-1759.  Genealogical Publ. Co., Baltimore, MD (Broderbund CD-195):

p. 135	CORBIN, BENJAMIN, m. Sarah Lye or Sye on 9 Dec. 1755 (131).
p. 620	SYER, SARAH, m. Benjamin Corbin on 9 Dec. 1755 (131).
p. 723	Church Records, MHS and HR. Except where otherwise noted, transcriptions of Church Records at the Maryland Historical Society were made by Lucy H. Harrison... 131 - Register of St. John's Protestant Episcopal Church Church. (sic) Index to Register made by Helen W. Brown. Typescript, MHS.

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

Corbin, Benjamin

Sye, Sarah

Dec 9, 1755

Baltimore Co.

2.  Richard B. Miller.  1985.  "Index to the Signers of the 1768 Joppa Petitions of Baltimore County, Maryland."  Maryland Genealogical Society Bulletin 26(4): 393-428 (Broderbund CD-208).

p. 393

By 1768 it had become apparent that the once thriving port of Joppa on the Gunpowder River was no longer capable of handling the commercial shipping for which the port facilities of the rapidly growing port of Baltimore Town on the Patapsco River were needed.  Largely as a result of this shift, it was obvious to most people that Baltimore was a far more appropriate seat for the county government.  The Assembly, however, with great fairness, chose to made this determination on the basis of a popular referendum, and ordered that petitions, both pro and con the change, be circulated throughout the county to be signed by all adult males over 18 -- virtually a census of adult county residents in 1768...

p. 399

Corbin, Beniamen, 533

Page numbers after individual's names are those of Appendix VII, Vol. LXI, Archives of Maryland, from which Miller compiled this alphabetical index. Those opposing the petition have the page number underlined.

3.  John Davis.  1996.  Baltimore County, Maryland, Deed Records, Volume Four: 1767-1775.  Heritage Books, Bowie, MD.

p. 207

2 Feb 1773, Eathan & Mary Loveall, of Baltimore Co., Maryland, to Benjamin Corbin, of same, £15, 109 acres. Signed Eathan Loveall. Wit: R. Richards and Abraham Stevens.

Benjamin's daughter, Sarah Ruth, married Nathan GREENLAND, grandson of Ethan & Mary (ALSPAUGH) LOVEALL.

4.  1790 Census Index/Images (online at Ancestry.com, Image #42 of 55 — list roughly alphabetized; extracted by Diana Gale Matthiesen):¤•

1790

MD

Baltimore Co.

Patapsco Lower Hundred

Roll M637_3

p. 206

Ln. 20

Benjam Corbin

2-1-3-0-0

These data indicate:

Gender and Type	Age Class	Therefore Born	Individuals Inferred
2 free white males	16 or over	in or bef. 1774	= Benjamin (b. 1730) = Asahel (b. 1774)
1 free white males	15 or under	1774-1790	= Nicodemus (b. 1778)
3 free white females	all ages	in or bef. 1790	= Sarah (b. 1736-40) = Sarah (b. 1774) = Keziah (b. 1785)

5.  1800 Census Index/Images (online at Ancestry.com):  not found, but our elderly couple is presumably living with one of their married children.

6.  1810 Census Index/Images (online at Ancestry.com):  not found, but our elderly couple is presumably living with one of their married children.  The only Benjamin CORBIN in Baltimore County is living in Mine Run Hundred (nowhere near Westminster) and appears to be our subjects' son.

7.  Letter, dated 19 Jun 1858, from Keziah (GREENLAND) SHANAFELT 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.  My two grandfathers (Moses Greenland and Benjamin Corbin), as I understand, lived about four miles apart.  The children of both families attended the same school just thirty miles this side of Baltimore near Winchester.  I never saw Grandfather Corbin.  He died when I was young.  I often saw his youngest brother William.  He came to Huntingdon.  While young he married Betsy Brown, sister to Mrs. Weaver's father.  He lived near Raystown Branch but died in Huntingdon.  Many of his descendants live there yet.  He had a sister, lived in Ohio.  Her family was said to be very wealthy.  I think her name was Marsh.  He had other brothers, too, but I am unacquainted with their history.  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.  She was married to a Baptist preacher at Millcreek, old uncle Samuel Lane.  She was his third wife.  He was struck with palsey while preaching in uncle Jacob Dean's house in Trough Creek Valley.  Father was present at the time.  Grandmother visited her friends in Huntingdon twice.  I well remember when she came to Father's, and likewise when she went away.  She died in Maryland at her son Joseph's merely with sickness, weakness and old age.  Most of them came to Huntingdon.  Uncle Elijah and Mycajah married sisters, Sarah and Mary Kelly.  They both died in Cassville since I came to Clarion. Joseph married Nelly Grover.  He lived near Winchester till after his mother's death, then moved to Bedford County until his wife's death, then went west.  Nichodemas married Mary Brown, moved to Ohio.  Uncle Ezekial and Asahal married sisters, Nelly and Lidaiu Corbin, Uncle William Corbin's daughters.  They both died at the ridges near Millcreek.  Their widows are living yet as far as I know.  Aunt Keziah Corbin, mother's only sister, still remains in Baltimore County near Winchester.  She visited her friends twice.  I remember well seeing her.  I think she never married.  If traveling had been as common and conveneint in my young days as it now is, I could give you a more full account of my friends.  The only mode of travel then was on horseback, of course not much of it done.

Jacob DEAN was Keziah's grand-uncle.

8.  Irene A. Nielsen.  1983.  "Query: CORBIN, LYE, KELLEY." 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?

Someone has mistaken "S" for "L," a very easy mistake to make with 19th-century handwriting.

9.  Broderbund.  World Family Trees.  Vol. 1, Pedigree #2346; Vol. 40, Pedigree #356 (has Benjamin born 1730).

10.  The Gene Pool — an entirely secondary source (online at Ancestry.com):

Name	Birth Date	Birth Place	Death Date	Death Place	Father	Mother
Benjamin Corbin	1722	Baltimore,, MD	16 June 1813	Westminster, Carrol, MD	Nicholas Corbin	Eleanor

11.  Public Member Trees (online at Ancestry.com).

12.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).