She is also sometimes listed as Josephine Brittania; and I’ve seen her name spelled with two ts, two ns, and both two ts and two ns.

Note: She’s not the same Britannia Andrews who is married to Sullivan Andrews in Buckfield (her name was Britannia Coolidge).

Was she Brittania J Page living in the family of Nathaniel W Foye in Hartford, ME at the time of the 1850 USC (age listed is 25, which is consistent with 1860 USC)? -- yes.

1860 USC for Franklin Plt lists the family:
Greenleaf Andrews ae. 40
Josephine B ae. 35 (b. abt 1825)
William G ae. 5 (b abt 1855)
Martin L Page ae. 15 (b abt 1845)
Rosalinda Page ae. 17 (b abt 1843)
Edwin V Page ae. 13 (b abt 1847)
Ina J Page ae. 11 (b abt 1849)

Who are these other Pages? I assume they’re related to BJ, but how? Why can’t I find them on the 1850 USC--or in 1870 for that matter? Who are BJ’s parents??

Martin L Page served in the Civil War, married Mira Rowe of Sumner and moved first to Iowa and then to New Mexico.

Edwin V fought and died in the Civil War. His military documents list his mother's name as Brittania J Harlow. So the children with Greenleaf in 1860 are apparently Brittania's children from a previous marriage (so her maiden name is not Page). One family tree on Ancestry.com lists Edwin’s birthplace as Milton Plantation, ME (source=Titus).

Rosalinda Page ae. 7 living with Reed family in Hartford, ME at time of 1850 USC, listed as Roselinda in 1860. She married Washington Huntoon Lovejoy (c.f.) in 1860, lived in South Paris, had a family, and died in 1933.

15 Nov 1868: Mr. Charles F. Smith & Mrs. Brittania J Andrews, both of Sumner, m in Sumner by Geo. Hopkins, JP. (From Marriage Returns of Oxford Co. Prior to 1892, p 173). Divorced Apr 1873.

Marriage to Addison W Harlow in Turner, 1 Sept 1873; int. 4 Aug 1873.

Tombstone in Minot/Turner notes that she was killed by a tornado in Pomeroy, Iowa and her body was returned to Maine for burial. She is buried in the Harlow Cem., Minot, Androscoggin Co., ME.

Note: Britannia was living with Nathaniel Foye’s family in Hartford in 1850 and they are also nearby in Sumner in 1860. Could Britannia be a Foye?

Update 8/5/12:

Yes, Britannia is a Foy(e): I bought the Hartford Vital Records from Picton Press, and she’s listed as Britana Josephina, a daughter of Nathaniel & Aurelia, as suspected. These records also note marriage int. between Britannia and Luther Page 11 Mar 1842. Mystery solved!