Isaac is said to not certainly be John's son, only that he is of the same generation as John's children.  In fact, I don't know what the evidence is for attaching any of the children to these parents.

Secondary sources also attach this son to John:
.  Joseph SIERS, b. bef. 1766; d. 2 May 1816, Jackson Co., IN; m. 3 Jan 1787, Greenbrier Co., VA/WV, Elizabeth McDADE 
but Joseph marries 13 years before any of John's other children marry, so I suspect Joseph is as likely John's brother as his son.

.  1796 Greenbrier County, (West) Virginia, Personal Property Tax List (online at New River Notes [link died]).

Name	White Tithes	Blacks 16+	Blacks 12-16	Horses
Siers, John	2	0	0	3

Also on same list:  Joseph SIERS.  The second white male tithable is presumably a son of John, now age 16 or older (males became tithable at age 16).  If we assume this son is James, then James had to be born in or before 1780.  And if Isaac is also son of John, he had to be under age 16 because he doesn't show up as a tithable; therefore, Isaac had to have been born in or after 1780.

.  Pam Shelton Anderson.  Ancestors of Pamela Loretta Shelton (Family Tree Maker User Page [link died]) and a gedcom at WorldConnect (online at RootsWeb.com).  Among her sources are:

.  Fauquier County, Virginia.  Deed Book 7, 1778-1783.  LDS FHL Film No. 0031586.  Quoting Anderson:

24 Sept 1781, John Siers sold land in Fauquier co to John Chick. This land had belonged to his grandfather John Siers this sale could also have been between John Siers son of John sen [Sr.] and not this grandson though the deed seems to indicate that it was a grandson of John sen [Sr.] who was the grantor of the deed.

To own/sell land, a male had to be 21 years of age.  This transaction fixes John's birthdate at no later than 1760 — if, indeed, this John is our subject.

.  Monroe County, West Virginia.  Deeds Index, Grantees, 1789-1969 (S-Z).  LDS FHL Film No. 0589736.  Quoting Anderson:

1785 John bought 200 acres of land on Indian Creek in what at that time was Greenbrier co VA. This is the earliest land record for him in Greenbrier/Monroe co area.

.  Larry Shuck.  1989.  Greenbrier County Records: Volume 2: Tax Records.  Iberian Publ. Co., Athens, GA. Quoting Anderson:

He appears in the Greenbrier co tax lists in 1786, 1792 and 1796.  In 1796, he has a second tithable in his household, probably his son James- this would seem to indicate that John was old enough to have a 21 year old son and makes it more likely that he was Joseph and James father.

Males became tithable at age 16, not 21.

.  Helen S. Stinson.  1988.  Greenbrier County, West Virginia, Court Orders, 1780-1850.  [Apparently self-published.].  Quoting Anderson:

p. 99	30 April 1790	John was defendant in a case against Samuel Caldwell, who John owed money.
p. 111	30 Mar 1791	Joseph Siers was a deft in a case against James Halstead, and John Siers undertook for the defendant.
p. 122	2 June 1791	John was deft in a court case against Roger Killpatrick, assignee of Hugh Paul.
pp. 160, 163, 175	1794-1795	[John] was involved as a defendant in a court case against William Hinchman.  Apparently this case escalated to the point that the sheriff was directed to take custody of John Siers in 1795, but the debt that he owed Hinchman was satisfied to the court's satisfaction.
p. 229	1798	In a Greenbrier co Court Case, John Siers was said to have intermarried with Sarah Paul, and the other Pauls listed may have been her siblings, John Paul, Isaac Paul, Gregory Paul, Daniel Paul, Jesse Paul, Mary Paul.
p. 91	28 Aug 1798	John Siers was deft in a case (which he lost) against James Kelly

.  Larry Shuck.  1988.  Greenbrier County Records: Volume 1....  Iberian Publ. Co., Athens, CA.  Quoting Anderson:

This same day [2 June 1791] he [John Siers] was also involved in a case with Joseph Siers against James Holstead.

. Greenbrier County, West Virginia. Deed Book 1, 1780-1789, p. 486.  LDS FHL Film No. 0593544.  Quoting Anderson:

p. 385	28 Apr 1792	John and Sarah sold property in Greenbrier co to Andrew Wilson.
p. 486	1795	[John] appointed Robert Rennick as his attorney to represent his interests in the estate of his grandfather of John Siers of Fauquier co.

. Greenbrier County, West Virginia. Deed Book 2, 1798-1803, p. 126.  LDS FHL Film No. 0593545.  Quoting Anderson:

p. 126

16 May 1798

John Siers sold livestock and personal property to James Graham in Greenbrier co... Rhoda Siers witness...

Rhoda had to be 21 to witness a deed, so she must have been born in or bef. 1777.  It does seem odd to me that John would have had his daughter witness a deed.  Could she have been a sister?  Or sister-in-law?

.  Monroe County West Virginia, County Court Records, Box 28, 1794-1858  (LDS Film #0239178, as quoted by Anderson):

This 29th day of September 1814 Between Sarah Seires widow and Relict of John Seires decd, James Seires, Moses Kean and Rhoda his wife - late Rhoda Seires, David Parsons and Jane his wife late Jane Seires and Charlotte Seires - Children and heirs of John Sieres decd Plantiffs

.  John Gott.  1993.  Fauquier County, Virginia, Court Records, 1776-1782.  Heritage Books, Bowie, MD, p. 38.  Quoting Anderson, with regard to Sarah PAUL, wife of John SIERS:

Possibly the Sarah Syars that received an allowance in 1778 as being the wife of a Continental Army soldier.

My understanding is that a woman would not receive an allowance unless she was a soldier's widow, and John SIERS, our subject, was alive at least as late as 1798.

.  Additional sources at WorldConnect (online at RootsWeb.com).