1.  Marriage Record:

2.  1840 Census Index/Images (online at Ancestry.com; Image #3 of 30; extracted by Diana Gale Matthiesen):

1840

PA

Lebanon Co.

Jackson Twp.

p. 122A

Ln. 21

Jacob Kapp

000 020 - 100 020

0200000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	20-29	1810-1820	= Jacob (b. 1815/6) = brother?
1 female	4 or under	1835-1840	= daughter? niece?
2 females	20-29	1810-1820	= sister-in-law? = sister? wife?
2	persons employed in agriculture

It's probable that Jacob is married by now and has a child by now, but if so, he lost them both and re-married by 1850, which seems likely given the age difference between Jacob and Sarah.  The marriage record would help, of course.

3.  1850 Census Index/Images (online at Ancestry.com; Image #1 of 8 — in the "Jackson2" group):  Jackson Twp., Lebanon Co., PA, p. 42B, 589/642, enumerated 24 Sep 1850, official enumeration date 1 Jun 1850 (extracted by Diana Gale Matthiesen):

Jacob Kapp	34	M		Farmer	7000	Pennsylvania
Sarah  "	26	F				"
Henry  "	2	M				"
Elias  "	1/12	M				"
John Wagoner	22	M		Laborer		"
Mary Wertig/Wertz	15	F				"

Listed next to Jacob's brother:  Henry KAPP.  Also in Jackson Township:  their father, Leonard KAPP, living with their brother, John KAPP.