Jane's father's will says that he had a daughter Jane Ann from his second wife Ann.
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We can see Jane in 1851 living at 20 Canon Street, Tower Hamlets, St George in the East, London Middlesex with her sister Isabella and brother-in-law Charles Francis:
Charles Francis Head 34 Victualler St George in the East
Isabella Mary Frances Wife 24 oxford
Jane Knibbs Sister 14 St George in the East
Mary Donovan Servant 30 Ireland
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I believe it is this Jane who we see in the 1861 census, living at 27 Colet(?) Place, Tower Hamlets, St George in the East, London, Middlesex with an Elizabeth Knibbs:
Elizabeth Knibbs Head 36 Seamans Wife St George in the East
Jane A Knibbs Sister 24 Fund Holder St George in the East

Elizabeth is believe to be a the wife of Edwin George Knibbs so in fact, a sister-in-law and not a sister. Edwin was the son (from a previus marriage) of Jane's father, Francis Knibs.