Marriages occurred in the county were the bride lived, so you might assume that Rachel's family lived in Loudoun in 1812. The list of Fulton's I previously sent from the 1812 Personal Property Tax Lists gives you an idea of who was living in Loudoun that year. No additional information was given on those lists.
The 1810 Loudoun Census shows:
p:l. 277A:32 FULTON, Ab’r 01010-10100-0/1
p:l. 275:13 FULTON, David 02101-20201-0/0
p:l. 307A:12 FULTON, Geo. 01001-21010-0/0
p:l. 281:13 FULTON, John 00020-20011-0/1
p:l. 279A:01 FULTON, Robert 22101-00011-0/5
p:l. 304:34 FULTON, Susan 01100-20201-0/0
The Genius of Liberty newspaper (began in 1817) indicates David Fulton died in 1823 at age 66 years
There is no mention in the Loudoun will books of any Rachel Madison or Rachel Fulton.