If Martha was born after 1843, her mother could have been Susan Brashears, who is listed as born in 1829 in Okla Falaya, Choctaw Nation, Mississippi. This would make Susan only 14, but would show the direct Choctaw lineage: Benjamin Brashears m. Illichihana abt. 1806 in Okla Falaya. His father was Robert Turner Brashears, b. abt. 1756 in Blandensburg, Prince George County, Maryland; m. Jane Apukshunnubbee (b. abt. 1775 in Okla Falaya, d. abt. 1822 in Mississippi) abt. 1786 in Okla Falaya; he d. abt. 1830 in Choctaw Nation, Mississippi. Jane's father was Apukshunnubbee, b. abt. 1740, m. Abu-to-nah (b. abt. 1760, d. after 1775) abt. 1774 and died abt. 23, 1824.
That would make (supposedly) Jane full-blood, Benjamin 1/2, and Susan 3/4, with Martha becoming 3/8.
Nettie Needham (nee McMurtrey) was recorded as 1/8, and her children as 1/16, so 3/8 could have been considered full and the logic is correct.
All the above is wildly speculative, and my attempt to find "my" Indians.