1.  (Daughter A) STEELY, b. bef. 1790; d.s.p. by 1800
  2.  (Daughter B) STEELY, b. bef. 1790; d.s.p. by 1800
  3.  (Daughter C) STEELY, b. bef. 1790; d.s.p. by 1800
  4.  (Son A) STEELY, b. 1774-90; d.s.p. by 1800
  5.  (Son B) STEELY, b. 1790-1800
  6.  (Son C) STEELY, b. 1790-1800
  7.  (Daughter D) STEELY, b. 1790-1800
  8.  (Daughter E) STEELY, b. 1790-1800
  9.  (Daughter F) STEELY, b. 1790-1800
10.  (Daughter G) STEELY, b. 1790-1800

1.  Marriage Record:

2.  1790 Census Index and Digital Images (online at Ancestry.com; Image #14 of 17; extracted by Diana Gale Matthiesen):

1790

PA

Mifflin Co.

p. 127

Ln. 1

Jacob Steely

1-1-4-0-0

These data indicate:¤•

Gender and Type	Age Class	Therefore Born	Individuals Inferred
1 free white male	16 or over	in or bef. 1774	= Jacob (b. ca. 1756)
1 free white male	15 or under	1774-1790	= Son A
4 free white females	all ages	in or bef. 1790	= Wife = Daughter A = Daughter B = Daughter C

Listed next to Jacob's father, Ulrich STEELY, on one side; and, in the other direction, listed next to Jacob's brother, Henry STEELY, who is listed four lines from Jacob's brother, Gabriel STEELY. 

3.  1800 Census Index and Digital Images (online at Ancestry.com; Image #17 of 27; extracted by Diana Gale Matthiesen):

1800

PA

Mifflin Co.

Derry Twp.

p. 496

Steely Jacob

20010-40010-00

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	9 or under	1790-1800	= Son C = Son B
1 male	26-44	1755-1774	= Jacob (b. ca. 1756)
4 females	9 or under	1790-1800	= Daughter G = Daughter F = Daughter E = Daughter D
1 female	26-44	1755-1774	= Wife

Based on these numbers, all four children born before 1790 are deceased, and they've had six new children since 1790.  I suppose it's possible.  The list was roughly alphabetized, so no neighborhood reconstruction is possible, but also in the township are Jacob's widowed mother and brothers, Gabriel and Lazarus.