Fanny Lucretia Thayer and Robert Bruce Arms married 25 December 1856, per the The Genealogical Record of the Arms Family...., page 28.
The "Memorial of the Thayer Name: From the Massachusetts Colony....." by Bezaleel Thayer indicates that ".... Fanny, married to Robert Armes and settled in Brattleboro, VT..." was the youngest child of Lewis and Laura Thayer. Siblings of Fanny: Titus, Thaddeus, Caroline, Adaline, and Elizabeth.
Apparently Lewis Thayer was the third child of Thaddeus and Rhoda Thayer. He married Laura M MILLER 15 October 1816 and settled in Dumerston VT. Farmer. Mrs. Laura Thayer was born 19 February 1795 and died 11 May 1851.
Pages 401 and 402 of the Thayer Memorial.
This is just one source - published in the 1880s (I believe.) Hope it gets you started!
Regards, Ann Morley Carmel
PS Lewis Thayer's Marriage Record in Dummerston, Vermont (Vermont Vital Records) records Laura's name as LUCENE Miller.