We know that George & Madgalene died in 1816 (in PA), so it is not out of reason that their son, Henry B. KAPP, came to Hamilton Co. where his uncle, Jacob KAPP, was already living.  The other likely alternative is that our subject is Jacob KAPP's son, not his nephew.

1.  Anon.  1966.  Marriage Index:  Ohio, 1789-1850.  Liahona Research, Orem, UT (Broderbund CD-400):

Capp, Henry B.

Butler, Lucy

16 Feb 1823

Hamilton Co.

2.  1830 Census Index and Digital Images (online at Ancestry.com; Image #35 of 62; extracted by Diana Gale Matthiesen):¤•

1830

OH

Hamilton Co.

Cincinnati Fifth Ward

p. 126

Ln. 5

Lucy Capp

020 000 000 - 020 111 001

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	5-9	1820-1825	= Son? = Son?
2 females	5-9	1820-1825	= Daughter? = Daughter?
1 female	15-19	1810-1815	=
1 female	20-29	1800-1810	= Lucy
1 female	30-39	1790-1800	=
1 female	60-69	1760-1770	=

There are obviously other women living in the household, besides Lucy, so it's hard to know which, if any, of the children are hers.

3.  1840 Census Index and Digital Images (online at Ancestry.com; Image #18-19 of 64; extracted by Diana Gale Matthiesen):¤•

1840

OH

Hamilton Co.

Cincinnati - 2nd Ward

pp. 71A-71B

Ln. 8

Lucy Capp

000 100 - 001 631

000/12/000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	15-19	1820-1825	= Son?
1 female	10-14	1825-1830	=
6 females	15-19	1820-1825	=
3 females	20-29	1810-1820	=
1 female	30-39	1800-1810	= Lucy
12	person employed in manufactures and trades

Lucy must be either taking in boarders or running a business with live-in female workers.