Mary MERTZ may have been John's second wife, in which case, only Diana is her child.

— born in Middlecreek Twp., Union [now Snyder] Co., PA:
  4.  Henry M. STRAUB, b. 1831/2 
  5.  Mary "Polly" STRAUB, b. 1833/4; m. Jeremiah HUBLER
  6.  Mary Ellen STRAUB, b. 1835/6; m. Reuben D. MILLER
  7.  Susannah STRAUB, b. 1837/8; m. William MILLER
  8.  John W. STRAUB, b. 1839/40
  9.  Robert STRAUB, b. ca. 30 Sep 1842; d. 25 Feb 1862; bur. Globe Mills Cem., Snyder Co., PA
10.  Charles M. STRAUB, b. Feb 1846 
11.  Diannah / Dianah / Diana STRAUB, b. 1848/9; m. Alfred MARBERGER / HARBURGER

1.  Marriage Record:

2.  1830 Census Index and Digital Images (online at Ancestry.com; Image #13 of 22; extracted by Diana Gale Matthiesen):¤•

1830

PA

Union [now Snyder] Co.

Centre [now Middlecreek] Twp.

p. 354

Ln. 19

John Stroup

101 010 - 100 010

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	4 or under	1825-1830	= Son A
1 male	10-14	1815-1820	= ?
1 male	20-29	1800-1810	= John (b. 1804/5)
1 female	4 or under	1825-1830	= Caroline
1 female	20-29	1800-1810	= Mary

The teenage male is too old to be their son.  List alphabetized.

2.  1840 Census Index and Digital Images (online at Ancestry.com; Image #5-6 of 20 under "Unknown Townships"; extracted by Diana Gale Matthiesen):¤•

1840

PA

Union [now Snyder] Co.

Middle Creek Twp.

pp. 261A-261B

Ln. 18

John Straub

120 001 - 211 010

0500000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	4 or under	1835-1840	= John W. (b. 1839/40)
2 males	5-9	1830-1835	= Henry (b. 1831/2) = George (b. 1829/30)
1 male	30-39	1800-1810	= John (b. 1804/5)
2 females	4 or under	1835-1840	= Susannah (b. 1837/8) = Ellen (b. 1835/6)
1 female	5-9	1830-1835	= Mary (b. 1833/4)
1 female	10-14	1825-1830	= Caroline
1 female	20-29	1810-1820	= Mary
5	persons engaged in agriculture

List roughly alphabetized.

3.  1850 Census Index/Images (online at Ancestry.com; Image #2 of 17 — mis-indexed "Straul"):  Middlecreek Twp., Union [now Snyder] Co., PA, Roll M432_831, p. 10B, 140/155, enumerated 15 Aug 1850 (extracted by Diana Gale Matthiesen):¤•

John Straub	45	M	Farmer	2500	Pennsylvania
Mary     do	44	F			do
Dianah   do	1	F			do
George P do	20	M			do
Henry M  do	18	M			do
Mary     do	16	F			do
Ellen    do	14	F			do
Susannah do	12	F			do
John W   do	10	M			do
Robert   do	8	M			do
Charles  do	4	M			do

Has Dianah been listed out of order deliberately or accidentally?  The only reason I can think of for her to have been deliberately listed first is that she is the only child of John and Mary, that is, that the other children were by a former wife of John's.  On the other hand, it would be unusual for a woman to have her first and only child at the age of 43.  Mary MERTZ had a sister named, Dianah, so that child is surely hers.

4.  1860 Census Images (online at Ancestry.com; Image #174 of 384):  Kremer P.O., Middlecreek Twp., Snyder Co., PA, p. 468, PN 64, 448/456, enumerated 20 Jun 1860 (extracted by Diana Gale Matthiesen):¤•

John Straub	55	M		Farmer	4000	1500	Pennsylvania
Polly   "	55	F					"
George  "	30	M		Day Laborer			"
Robert  "	27	M		Do			"
Charles "	24	M					"
Susan   "	21	F					"
Diana   "	11	F					"
Mary Moore	7	F					"

Robert should be 17, not 27; and Charles should be 14, not 24.  I don't know who Mary MOORE is.

5.  1870 Census Index/Images (online at Genealogy.com; Image #2 of 16):  Kremer P.O., Middlecreek Twp., Snyder Co., PA, Roll 1451 (Book 1), p. 78B, 14/14, enumerated 15 Aug 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):¤•

Stroub John	66	M	W	Gentleman	6000	400	Pa
"   Mary	64	F	W				Pa

Listed next to son, Charles M. STROUB. 

6.  1880 United States Census and National Index (LDS Family History Resource File: CD-ROM Library):  not found, as expected.

7.  Globe Mills Cemetery (online at the Snyder County PAGenWeb site).  This is a compilation of readings from three sources:  Wagenseller (1904), Finsterbush (1993-1998), Finsterbush & Denius (1998).  Unfortunately, they've lumped all the readings together, so you can't tell which reading is from which source.  Also unfortunately, they don't give full citations to the sources, only name and date, so while I happen to have a copy of Wagenseller, I haven't a clue how to find or get copies of the other sources.

Straub, John, Feb. 4, 1804 - June 4, 1877; 73 y., 4 m. Isa. 3:10.

Straub, Polly, w/o John, Jan. 14, 1805 - Feb. 7, 1877; 72 y., 21 d.

Straub, Robert, s/o John & Polly, Sept. 30, 1842 - Feb. 25, 1862; 19 y., 4 m., 25 d.

The fact that Robert's mother was "Polly" may only mean that both of John's wives (if he had two) were named Mary.

8.  Birthdate calculated using Ben Buckner's JavaScript Birthday Calculator.

Entered		Result
4 Jan 1877	0d 4m 73y	4 Sep 1803
7 Feb 1877	21d 0m 72y	17 Jan 1805
25 Feb 1862	25d 4m 19y	31 Sep 1842
(Calculated birthdates should always be prefaced with "ca." — you can't be certain the person who calculated their age did it correctly.)

As the census tells us John was born in 1804 or 1805, the error here appears to be in John's age at death.  For Robert, the error in the result (Sep only has 30 days) means this date is impossible based on the age at death, that is, the error is really in the number of days in the age at death.

9.  Will of John Straub (photocopy generously provided by Debbe Hagner).  Summary by DGM:

Testator:  John STRAUB of Middlecreek Township, Snyder County, State of Pennsylvania Wife:  Polly STRAUB Sons:  George P. STRAUB Daughters:  Mary Ellen STRAUB (w/o Reuben D. MILLER)