Mary Lou,
I think the Waddle family that John is from may have been in the Pittsburgh vicinity by 1850, since there doesn't appear to be a John Waddle age 14 in Mercer County 1850 Census. They do not appear directly related to the Waddle families in Mercer County in 1850. The Reynolds/Rannals family you are searching for may also be in the Pittsburgh area or somewhere else.

The Mercer County 1850 census index listing may be of some help at:
http://www.rootsweb.ancestry.com/~pamercer/PA/Census1850/185...
There is an online transcription of the 1850 census for the county, and you may be able to do some research there.

We know of Susan Campbell, b. about 1828. There may be other sisters that we don't know about, such as perhaps a Jane:
Reynolds Wilson Sndy Crk 12
2 519 526 RAYNOLDS, Walcott 51 M Farmer 800 N.Y.
3 519 526 RAYNOLDS, Elizabeth 49 F Pa.
4 519 526 RAYNOLDS, Madison 16 M Farmer Pa.
5 519 526 RAYNOLDS, Angeline 21 F Pa.
6 519 526 RAYNOLDS, Elizabeth 11 F Pa. S
7 519 526 RAYNOLDS, Emily 8 F Pa. S

8 520 527 REYNOLDS, Wilson 24 M Farmer 150 Pa.
9 520 527 REYNOLDS, Jane 32 F Pa.
10 520 527 REYNOLDS, Angeline 1 F Pa.
11 520 527 LAMB, Joseph 49 M Sadler Pa. Pauper
Note: Jane Reynolds MIGHT be a Campbell. My William Campbell is 33, and is believed to be Eliza, Susan, and John’s brother, but this is a long way from Delaware and Lackawannock Townships.

Here is another possibility if Susan had a twin named Martha, or one is about to turn 23 or the ages are slightly incorrect for one or both:
Reynolds Charles R. Pymatnng 19
7 138 139 REYNOLDS, Charles R. 20 M Sawyer Ohio E
8 138 139 REYNOLDS, Martha J. 22 F Pa. E
[E represents cannot read or write, and may explain why there are spellings of "Rannals" instead of "Reynolds"?]

I would think that if Thomas Reynolds is a nephew that had been adopted, it would be more logical to have Eliza take in one of Eliza's sister's children, rather than John's sister's son.

John Waddle had sisters Sarah, Sophia, and Belle. I do not have a full listing of Eliza Campbell's sisters.

Good luck and let me know if you come up with any of Eliza's other sisters.

Paul