The following are census records.I could not find Martha in 1870.There is no record that I could find of her marrying a Nathaniel Davis, Branson Boling married 9/24/1872 in Clay Co., Ind was the only marriage record I found.
1850 Praire, Henry Co., Ind Nathaniel Davis 45 VA farmer Abigail 42 KY Amanda L. 22 KY (died 1921 Edgar Co., Ill, married John L. Bouslog 1826-1900, 14 children, public tree at ancestry.com) William J. 19 Ind Nathaniel 17 Ind David A. 15 Ind Nimrod L. 13 Ind Mary L. 10 Ind MARTHA J. 9 Ind Rebecca L. 6 Ind Ann Sophrona 3 Ind
1860 Jackson, Parke Co., Ind. Nathaniel Davis 56 KY farmer Abagail 54 KY Mar 20 KY school teacher JEMIMA 18 Ind Rebecca 16 Ind Ann 12 Ind
1880 Van Buren, Clay Co., Ind. Isaac Thomas 61 KY (parents: KY) works on farm Martha J. 39 b. Ind (f) Va (m) KY Joseph 13 Ind (grandson) Rosannah 1 Ind (niece)
1900 Van Buren, Clay Co., Ind. George W. Thomas 5/1854 Ind (f) Ky (m) Ind Jeanette H. 5/1861 (married 23 yrs, 9 children, 4 living) b. Ind (parents: Ind) Bertha i. 12/1883 Ind Laurence E. 8/1890 Ind Lillie V. 1/1894 Ind Floyd C. 4/1896 Ind Martha J. Thomas (mother-in-law) 12/1841 widow 2 children, 1 living b. Ind. (f) VA (m) KY GEORGE W. THOMAS WAS THE SON OF ISAAC THOMAS AND ELIZABETH THOMAS AS SHOWN IN THE 1870 VAN BUREN CO., IND CENSUS - I tried to track back on Jeanette as a Davis and as a Thomas without luck
1910 Van Buren, Clay Co., Ind George W. Crist 76 b. OH (parents: VA) (1st marriage, 2 years) retired Martha J. 69 (1st marriage, 2 yrs) b. Ind (f) VA (m) KY
1920 Van Buren, Clay Co., Ind George W. Crist 86 b. OH (parents: VA) no occupation Martha J. 78 b. Ind (f) Va (m) KY
If Martha was married to a Davis circa 1861 or late 1860, then Jeanette/Janette should have shown on the 1870 census - she does not show as Davis nor as Thomas nor as Boling.
I tried a search of all Indian for: Martha J. b. 1841/42, Martha Jemima b. 1841/42; Jemima b. 1841/42 and did not find her in 1870.I also searched for an Albert Davis with mother of Martha J., Martha or Jemima and found nothing.
It appears that Martha married George Crist circa 1908.Since in 1900 Martha answers that she has had 2 children but only 1 is living it is presumed that Albert is deceased.The Jeanette Thomas shown is most likely Martha's daughter and it is by coincidence that her daughter married the son of her husband, Isaac Thomas but again, I could not find her in 1870.