Unfortunately, I cannot identify your "Mary Ann Kephart" though I feel confident she is somehow related to my line.Here are the children of Andrew Kephart b.abt 1788, d.bef.1860: John A Kephart, b. 1812; Mary Ann Kephart, b. 1815; Catherine, b. 1816 ; Rachel, b. 1822; Isaac, b. 1824; Nancy Jane b. abt 1827; Jacob, b. abt 1829; George, b.abt 1830; Lavina, b.abt 1834; Permina, b.abt 1838; Susanna, b.abt 1840, and Tamer, b.abt 1843.
Yes, that is a HUGE family.
The only other Mary that I have would be the daughter of John A Kephart b.1812; his daughter Mary was born in about 1835 (from census), so it seems unlikely that she is the one you are seeking.