Other children said to be:
.  James SIAS/SIERS, b. bef. 1755 
.  John SIAS/SIERS, Jr.
but they are not mentioned in John's will.

1.  Census Index:  Colonial America, 1607-1789 (Broderbund CD-310):

1762 Rent Roll

John Sias of Powell

VA

Prince William Co.

2.  Ronald N. Hambrick Family History (online at WorldConnect).  Hambrick states:

The records of Fauquier County, of which Warrenton is the seat of government, disclose that one, Benjamin Hamrick, was married and living in that county prior to 1773.  That is the date of the execution of the Last Will and Testament of John Sias.  John Sias obtained a land grant of 639 acres of land from Thomas Lord Fairfax in Prince William County, Virginia, in 1740,4 the same year Patrick Hamrick obtained title to his land in Prince William County. In 1779, the Last Will and Testament of John Sias was admitted to record in Fauquier County.5 That Will gave, "to my daughter, Mary Hamrick, five pounds current money." At November Term of Court, 1779, an order was entered summoning, "Mary Hamrick, wife of Benjamin Hamrick, to appear to context the proof of the Will of John Sias." 4Book E, page 194, Virginia State Land Office, Richmond, Va.  5Will Book 1, page 386.

3.  Deed Abstract courtesy of R. L. Cogar.

1757 Jul 13 -- Simon Miller of Prince William Co VA, survey by William West, 249 acres in Prince William Co VA, beginning at a corner to Jacob Holtsclaw and with his line ... a flat rock his corner ... oaks on the top of a mountain ... chestnut corner to said Holtsclaw ... chestnut on a hillside ... white oak corner to George Roggers ... maple corner to said Roggers and in the line of JOHN CYUS, then with CYUS's line ... hiccory corner to said CYUS ... oak on a stony point ... white oak all corners to said JOHN CYUS's land ... oak corner to FISHBACK and said Jacob Holtsclaw, then with their line.  VA NN Grants I, p. 33. (RLC)

4.  Descendants of Patrick Hamrick, Sr. (online at WorldConnect) by J.R. McKinney.  In the notes for Benjamin & Mary (SIAS) HAMRICK:

John Sias Will, dated 6 September 1773.  Wife Mary Sias, "a full and sufficient maintanance out of my estate during her Life or widowhood to be given to her by my Executors as they shall think suitable for her use.  The tract of land now mortgaged to William Carr be the same more or less to Thomas Chapman he paying William Carr's debt due on the said land. Also I give said Thomas Chapman my negro man slave named Sam and the rest of my estate.  To my daughter Mary Hamrick - 5 Pounds. Abstracts of Fauquier County, Virginia. Exor: Thomas Chapman, William Carr. Wit: James Muschett, James Gwathin, John Tebbs. Wills, Inventories, and Accounts. 1759-1800. No other child mentioned. Will proved 22 Nov 1779.

5.  John Gott.  1980.  Abstracts of Fauquier County, Virginia, Wills, Inventories, and Accounts, 1759-1800.  Genealogical Publ. Co., Baltimore, MD (as posted by Pam Shelton-Anderson at GenConnect: Sires: Wills).

John Sias Will, dated 6 Sept 1773 Daughter Mary Hamrick - 5 pounds Wife Mary Sias, "a full and sufficient maintenance out of my Estate during her life or widowhood to be given her by my Executors as they shall think suitable for her use. The tract of land now mortgaged to William Carr be the same more or less to Thomas Chapman he paying William Carr's debt due upon the said land. Also I give the said Thomas Chapman my Negro man slave named Sam and all the rest of my estate. Exor: Thomas Chapman, William Carr Signed: John Sias Wit: James Muschett, James Gwathin, John Tebbs Proved 22 Nov 1779

6.  Lease to George Leach (Fauquier Co., VA, Deed Book 1, 1759-1763; LDS FHL Film #0031583; online at GenConnect: Sires: Deeds; transcription by Pam Shelton-Anderson).  On 23 Sep 1762, John Sias leased 150 acres to George Leach.

John Sias of Fauquier county, colony of Virginia...  Signed: John Siers George Leach Witnesses: Richard Healey, James Siers, Anne Champe

7.  U.S. Geological Survey.  Geographic Names Information Service (GNIS online database).  No Powell in Prince William Co., VA.

8.  Merriam-Webster.  1977.  Webster's New Geographical Dictionary.  G. & C. Merriam Co., Publ., Springfield, MA.  No Powell in Prince William Co., VA.

9.   AniMap Plus: County Boundary Historical Atlas: with "SiteFinder," U.S. Place Name Database and U.S. Cemetery Database. (On CD.) Gold Bug Software, Alamo, CA.  No Powell in Prince William Co., VA.