To have been Sara's mother, Anna couldn't have been born any earlier than 1730 and, more likely, she was born several years later.

1.  Ralph Beaver Strassburger (William John Hinke, ed.).  1934. Pennsylvania German Pioneers: a Publication of the Original Lists of Arrivals in the Port of Philadelphia from 1727 to 1808.  2 vols.  Norristown, PA (reprinted 1966ff by Genealogical Publ. Co., Baltimore, MD; Broderbund CD-267):

p. 472	[List 174 C] At the Court House at Philadelphia, Friday, the 4th 8ber 1751. Present: . . . The Foreigners whose names are underwritten, imported in the Ship Queen of Denmark, George Parish, Commander, from Rotterdam & Cowes, did this day take & subscribe the usual Qualifications. No. 99. Whole Frieghts 251...
p. 473	Johannes Strub

2.  Julius Friedrich Sachse.  1897-1905.  "The Records of St. Michaelis and Zion Congregation [German Evangelical Lutheran Congregation] of Philadelphia: Births, Baptisms, and Marriages."  Pennsylvania German Society Proceedings and Addresses VII(1897): 534-576; VIII(1898): 199-222; IX(1899): 449-468; XIV(1905): 1-140 (reprinted in 1983 as pp. 486-900 in Volume I of Pennsylvania German Church Records by Genealogical Publ. Co., Baltimore, MD; Broderbund CD-130):

p. 520	ANNO 1751...
p. 528	Keppler, Juliana Cathrina, dr. Sebastian Keppler and w. Anna Elisabeth; b. Oct 28; bap. Nov. 3; sp. Juliana Schmidtin.  Johan Strub, Anna Cathrina Strubin.
p. 556	ANNO 1754...
p. 557	Reinhard, Anna Cathrina, dr. Christian Reinhard and w. Veronica; b. Feb. 4; bap. Feb. 10; sp. Johannes Strupp (ref.) and w. Anna Cathrina (luth.).
p. 565	Strob, Catharina, dr. Johannes Strob and Cathrina; b. Sept. 11; bap. Sept. 29; sp. Magdalena Ellen, Cathrina Schmidtin.

3.  John T. Humphrey.  1991-1998.  Pennsylvania Births.  14 vols.  Self-published, Washington, DC (Broderbund Family Archives: Birth Index: Southeastern PA, 1680-1800, CD-196):

Child	Catharina Strob	Maria Magdalena Strupp	Jacob Strub	Sara Strup
Birth/Baptism	11 Sep 1754	11 Jul 1761 (Baptism)	3 Aug 1770	10 Jan 1773
Location	St. Michael's and Zion Lutheran Church	First Reformed Church	First Reformed Church	First Reformed Church
County	Philadelphia	Philadelphia	Philadelphia	Philadelphia
State	Pennsylvania	Pennsylvania	Pennsylvania	Pennsylvania
Father	Johannes Strob	Johannes Strupp	Johannes Strub	Johannes Strup
Mother	Mrs. Cathrina Strob	Mrs. Anna Cathareina Strupp	Mrs. Cathrina Strub	Mrs. Cathrina Strup

4.  F. Edward Wright.  1994.  Early Records of the First Reformed Church of Philadelphia. Vol. 1: 1748-1780; Vol. 2, 1781-89.  Family Line Publ., Westminster, MD  (Broderbund CD-166):

BAPTISMS BY THE REV. JOHN CONRAD STEINER, 1759-1762... Mary Magdalene, daughter of John Strupp and Anna Catherine, bapt. July 11, 1761.  Spon:  John Franck and Christine...

BAPTISMS BY THE REV. CASPER WEYBERG, 1763 - 1774... Catherine, daughter of John Krebel and Elizabeth, b. July 25, 1769, bapt. August 24, 1769. Spon: John Strub and Catherine...

BURIALS BY THE REV. CASPER WEYBERG, 1764 - 1780... Susanna Strup, age 1 year, 1 month, 3 weeks, buried August 12, 1769. [Is she our subject's daughter or granddaughter?]

BAPTISMS BY THE REV. CASPER WEYBERG, 1763 - 1774... Jacob, son of John Strub and Catherine, b. August 3, 1770, bapt. September 8, 1770. Spon: The Parents... Sarah, daughter of John Strup and Catherine, b. January 10, 1773, bapt. February 14, 1773. Spon: Elizabeth Krebel...

BAPTISMS BY THE REV. CASPER WEYBERG, 1763 - 1774... John Andrew, son of Jacob Zing and wife, b. ____, bapt. July 30, 1774. Spon: John Strub and wife...

BURIALS BY THE REV. CASPER WEYBERG, 1781-1785... John Strup, age 55 yrs., 7 mos., bur. April 17, 1781...

BAPTISMS BY THE REV. CASPER WEYBERG, 1781 - 1789... John, son of Martin Wall and Christine, b. Aug. 14, 1784, bapt. Sept. 26, 1784. Spon: John Strub and Elizabeth Stocky... Maria, daughter of Henry Strub and Susanna, b. Jan. 14, 1785, bapt. March 25, 1785. Spon: The Parents and Catharine Strub...

If our subject died in 1781, then the John "STRUB" in 1784 must be someone else.  A big problem with all of F. Edward Wright's compilations is that, instead of transcribing the names verbatim, he translates them to English (e.g., Johannes to John, Maria Magdalena to Mary Magdalene, etc.), so you still have to consult the original — or a faithful transcription — to know what was actually written.

5.  Julius Friedrich Sachse.  1983.  "The Records of St. Michaellis and Zion Congregation  [German Evangelical Lutheran Congregation] of Philadelphia."  Vol. I, pp. 486-900 in Pennsylvania German Church Records from The Pennsylvania German Society Proceedings and Addresses. Genealogical Publ. Co., Baltimore, MD (Broderbund CD-130):

Anno 1751... Keppler, Juliana Cathrina, dr. Sebastian Keppler and w. Anna Elisabeth; b. Oct. 28; bap. Nov. 3; sp. Juliana Schmidtin. Johan Strub, Anna Cathrina Strubin.

6.  Friedrich S. Rothenbueler.  1987.  "Baptisms from the Church Book of the German Reformed Church of Philadelphia."  Pennsylvania Genealogical Magazine XXXV(1): 9-16 (Broderbund CD-213):

p. 13

*11 December Johan Martin & Anna Maria, twins, (died 12 December 1762), children of Jacob Gress of the Palatinate [Jacob Gross came on the ship Halifax, from Rotterdam, 22 September 1751. Egle, 355] & wife Anna. Sponsors: Johannes Straub of the Palatinate [? Johann Georg Sabb, came on the ship Patience from Rotterdam, 9 September 1751. Egle, 331] & wife Anna Catherina. Johann Heinrich Schuhman of Hackenburg & Maria Scheider.

I don't believe for a minute that Johannes STRAUB is Johann George SABB.  But could Johannes STRAUB be our subject?

7.  Census Index:  Colonial America, 1607-1789 (Broderbund CD-310):  not found, at least not as any obvious variation of our subjects' names who are not otherwise accounted for or unlikely.

8.  1790 Census Index/Images (online at Ancestry.com; Image #15 of 23; extracted by Diana Gale Matthiesen):¤•

1790

PA

Philadelphia Co.

Northern Liberties

Roll M637_9

p. 201

Ln. 24

Catharine Stroop

2-0-2-0-0

These data indicate:

Gender and Type	Age Class	Therefore Born	Individuals Inferred
2 free white males	16 or over	in or bef. 1774	= ? = Jacob? (b. 1770)
2 free white females	all ages	in or bef. 1790	= Catharina (b. 1730s) = Catharina (b. 1754)

Is the head-of-household our subject's widow or his unmarried daughter?  I think it must be his widow because, if not, one of the males would have been made head-of-household. 

9.  Frank F. Bell II.  1960.  BELL-SHARPE: containing Genealogical and Some Biographical Sketches and Illustrations on … Allied Families to BELL: ATTMORE, EDWARDS, GEORGE, SAUSSAT, RAKESTRAW, STROOP, STULL, ZANE… Unpublished typescript, Dallas, TX (online at HeritageQuest at Genealogy.com).  See excerpt.