I wonder if this solves it?
(1861 census)
Jesse Mogg, Head, 43, Baker, born Haselbury Brian, Dorset
Elizabeth Mogg, Wife, born Henstridge, Somerset
William Mogg, 15, Son, Bakers Apprentice, born Lydlinch
Eliza Mogg, 11, Dau, born Lydlinch
Charles Moss, 8, Son, born Lydlinch
Sarah Ann Mogg, 2, , Daughter, born Lydlinch
I wonder if Sadie and Eliza were Sydenham's nieces (even with Eliza's age being a little off). If they were Sydenham's children they don't seem to have ever lived with him and I would have expected perhaps to find them with him at least once. In 1911 Eliza has a son called Jesse (named after grandfather perhaps?)