Some secondary sources have the Eva Maria Magdalena KAPP who married Joh. Georg DIEFFENBACH as the daughter of Joh. Andreas KAPP & Regina Sophia MUELLER.  Without primary documentation, I can't tell which is correct.

.  William Henry Egle, ed.  1890.  Names of Foreigners Who Took the Oath of Allegiance to the Province and State of Pennsylvania, 1727-1775, with the Foreign Arrivals, 1786-1808.  Pennsylvania Archives, Vol. XVII, Second Series, Harrisburg, PA (reprinted 1967ff by Genealogical Publ Co., Baltimore, MD; in 2000 as Broderbund CD-512):

p. 141	LIST OF FOREIGNERS IMPORTED IN THE SHIP CHARMING NANCY OF LONDON, CHAS. STEDMAN, MASTER, FROM ROTTERDAM. QUALIFIED OCT. 8, 1737...
p. 142	George Titus Cap... [The original list is herewith given.] Men's Names.
p. 143	Georg Fyt Cap...
p. 144	Women and Children's Names...
p. 145	Georg Martin Capp...

Georg "Fyt" is Martin's brother, Georg Viet.

.  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. 188	[List 49 A] A List of People of the Charming Nancy, Charles Stedman, Comr from Rotterdam. [Qualified October 8. 1737.]...
p. 189	Georg Fyt Cap... WOMEN & CHILDRENS NAMES...
p. 191	Georg Martin Capp... Philadia, Oct. 8th, 1737. A true List. Charles Stedman.

.  Israel Daniel Rupp.  1876.  Thirty Thousand Names of German, Swiss, Dutch, French, and Other Immigrants in Pennsylvania from 1727 to 1776.  (repub. 1931, Leipzig; republ. 1965, Genealogical Publ. Co., Baltimore, MD; Broderbund CD-267):

p. 110	49) Oct. 8, 1737. Palatines imported in the ship Charming Polly, of London, Charles Stedman, Master, from Rotterdam,
p. 111	last from Plymouth.—One hundred and five men, twenty-five boys, and one hundred and seven women and girls.--237 passengers.—(Editor)...
p. 112	Georg Titus Cap... Under sixteen... Georg Martin Cap...
p. 465	APPENDIX No. XIV. NAMES OF EARLY SETTLERS IN TULPEHOCKEN, BERKS AND LEBANON COUNTIES. — As early as 1723, thirty-three families of Germans, who had come to New York 1710, settled in Schoharie 1713, left there and located on Tulpehocken Creek; among these were... In 1728, others from Schoharie settled here... Andreas Kapp...
p. 466	The first settlers in Tulpehocken were Lutherans and German Reformed.  Rev. Tobias Wagner was the first pastor of the Lutheran Congregation.  The following is a List of the members of the Tulpehocken Church, from 1743 to 1746: Georg Graf sen., Friederick Kapp, Andreas Kapp... Andreas Graf...
p. 467	Joh. Martin Cass, Geo. Vitus Cass, Joh. Michel Kapp... Johannes Graf... Georg Graf jr...

The big question is:  Is the Andrew KAPP who came to Tulpehocken in 1728 the same one who appears there in the 1740s?  The five KAPPs in Tulpehocken in the 1740s are considered to be brothers ("Cass" is obviously a misreading of "Cap"), who immigrated to Pennsylvania in the 1730s.  So if, as it seem probable, these are two different Andrew KAPPs, how does the one from New York relate, if at all, to the ones there in the 1740s?  And, in any case, how are we going to separate their children?!

.  Guiseppi, M.S., ed.  1921.  Naturalizations of Foreign Protestants in the American and West Indian Colonies. Huguenot Society Publ., London (online at Ancestry.com):

Name	Township	County	Date
Martin Kapp	Heidleberg	Lancaster	15 Sep 1751
Eva Margaretta Kap(p)	Heidleberg	Lancaster	15 Sep 1751

.  Land Records: Bucks and Lancaster Counties, Pennsylvania, 1682-1825 (Broderbund CD-652):

Record ID	Date	Residence	Book-Page	Remarks
41144	28 Mar 1748	Lancaster Co.	B-548	Deed
Property	400 acres in Heidleberg Township
Grantor	Widow Mandoline Groffe
Grantees	Andrew Groff, George Groffe, Hans Groffe, Sebastian Groffe, Leonard Holstein, Barbara Holstein George Emort, Eva Maria Emort Frederick Kapp, Eva Maria Kapp Martin Kapp, Margaret Kapp
Grantee's Wife	Catherine Groffe