From the 1850 Loudoun Co Census, page 315B:
Horseman, Stephen, 70, M,Georgetown DC
Horseman, Sally, 45, F, Georgetown DC
Horseman, Ann, 23, F, Georgetown DC
Horseman, Jane, 19, F, Georgetown DC
Horseman, Kitty (insane), 50, F, Georgetown DC
From the 1860 Loudoun Co Census:
HORSEMAN, Sarah (60y), HORSEMAN, Martha J. (30y), Samuel E. (27y farm hand), p. 325:005, Guilford Station
From the 1870 Loudoun Co Census:
HORSEMAN, Sarah (67y), Jane (42y), Martha (7y), Thomas (16y works on farm), George W. (2y), 013:025, Eastern District-Guilford
From “Loudoun Co Death Register, 1853-1896” by Frain and Hiatt:
HORSEMAN, Sarah, white female, d. 9 Nov 1886, nr Farmwell, pneumonia, 82-0-0, parents: Stephen & Mary Horseman, b. Loudoun, unmarried, source: Wm. H. Horseman, brother, 1st Dist.
It does not appear that Martha J. Horseman ever married.
If you have questions about Loudoun Co records, please email me directly.