We know know she was not the daughter of Henry Thorpe!
He maiden Name Ellis [given in marriage record] Married 1st on 1 May 1677 in Billerica, MA Benjamin Bullard, son of Robert Bullard and Anne Martyn Thorpe [Billerica VR 231]
Theory:
She is the daughter of Richard Ellis and Elizabeth French of Dedham born 23 May 1651. Benjamin Bullard was also from Dedham and likely knew the family. Elizabeth Ellis is the only recorded Elizabeth Ellis to be the correct age:
Dedham (Massachusetts). Town Clerk; Julius Herbert Tuttle; Benjamin Fisher; and Don Gleason Hill. The early records of the town of Dedham, Massachusetts. (Dedham, Massachusetts: Dedham Transcript Press, 1886-1936), p. 5. 1651.
Ellis: Elizabeth, the Daughter of Richard & Elizabeth Ellis, was borne the 23 of the 5 m'o.
Problem: why was she married in Billerica???
Can we tie Elizabeth of Dedham to Billerica? Otherwise this theory is weak. Yes we can:
a) She and Benjamin Bullard were both from Dedham
b) The only other Ellis marriages in Billerica besides theirs were
1) Eleazar Ellis her nephew;
2) Mary (Sibley) Ellis wid of her brother Ebenezer;
3) Ruth (Morse) Ellis widow of her uncle Joseph Ellis.
c) Who Else was in Billerica? The Entire French Family!!! In other words her mother's family.
Given her age is correct for this marriage and that her family has links to both Dedham and Billerica I think this conclusions is very convincing.