I think Unknown-401995 might be Elizabeth Atkinson (or Adkinson). This is based on:
Her daughter, Charlotte Ellis (Small-4329), in the 1851 census, is with her Aunt, Ann Glover, a widow, also born in Thornton, Lincolnshire in ~1778.
There were two sisters, of the correct names and yob, baptised in Thornton Curtis Elizabeth and Ann Atkinson (or Adkinson in one record), both daughters of William Atkinson/Adkinson and Mary.
(note: Thornton Curtis is not the same place as Thornton; was it abreviated in the census returns, or were Elizabeth Small and Ann Glover really born in Thornton, rather than Thornton Curtis? If the latter they can't be the Atkinson sisters.)
Could anyone help confirm or refute this theory?