The trouble with the later birthdates for John is that we then have a 30-year span of fecundity for Phoebe, which is unrealistic in those times.  Of course, it may be that the births of the eldest children are estimated too early.

One source also gives them a daughter:
.  Tamesin SIAS, b. 3 Sep 1749 
apparently because Tamisen is said to be mentioned in Samuel's will.  But another source says she was merely a favored niece, not a daughter, and that she is the daughter of Samuel's brother, Capt. Joseph SIAS.  Or were there two Tamisen's? 

1.  Marriage Record:

2.  1790 Census Images (online at Ancestry.com, Image #2 of 2; extracted by Diana Gale Matthiesen):¤

1790

NH

Strafford Co.

Lee Twp.

p. 383

Ln. 23

Phepe 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	= Phoebe = ?

I cannot be certain this Phoebe SIAS is Samuel's widow, but it seems likely.

3.  LDS.  Family Search: Internet Genealogy Service: Ancestral File (AF).

4.  LDS.  Family Search: Internet Genealogy Service: International Genealogical Index (IGI).

Tamesin SIAS Christening: 3 Sep 1749, Durham, Strafd, New Hampshire Father: Samuel SIAS; Mother: Mrs. Phoebe SIAS Source: patron submission

Tamesin SIAS Birth: 3 Sep 1749, Durham, Strffr, New Hampshire Father: Samuel SIAS; Mother: Phoebe Source: patron submission

5.  WorldConnect (online at RootsWeb.com).

6.  Descendants of John Sias Sr. by Garey L. Marshall.

7.  Ancestry World Trees (online at Ancestry.com).