Well strictly speaking there were two Eli Northams. The second was born in 1883 in Yarcombe the son of James Northam. That family moved to Wales and Eli junior married in Pontypridd in 1908.
Anna Grabham the wife of Thomas Grabham is consistently with her husband and family on the 1871 through to the 1901 census. So she cannot be the same Hannah Grabham with Eli on the 1891 census who he later marries. Have you purchased their marriage certificate because that would give you details of her father and help you place her on an earlier census?
How many times did Eli marry? I have found three
1870 to Sarah A. Nills in Chard
1896 Anna Grabham in Axminster
1907 possibly Grace Lock in Honiton.
Yet on the 1871 census Eli (or his wife) have a daughter born 1857 Mary Northam? So was the marriage to Sarah a very late marriage or was Eli married before? Do you have this marriage certificate as it should say whether Eli was a bachelor or a widower. Sarah would have been very young to have been Mary's mother 15/16. Do you have Eli on a census before 1871?
Hannah Grabham's age seems a moveable feast.
On the 1891 census her birth would be circa 1855
1901 census circa 1866
And on her death in Honiton in 1906 circa 1860
What was her age on marriage?
On the 1891 census Hannah's relationship to Eli was given as 'niece'. She could not have married him if she was his niece by blood, so she would have to be either Sarah's niece or a previous wife's niece. Even so to marry his dead wife's niece was actually prohibited legally until 1931 and of course to marry a niece by blood is still prohibited.
On the 1881 census the only female Grabham either Hannah/Anna/ Ann(ie) within the age range and born Yarcombe is Annie born about 1866 and with her parents in Upottery
RG12 3133 folio 71 Woodcott, Upottery, Devon
Abraham Grabham abt 1843 Yarcombe, Devon, Head Married Agricultural labourer
Emily Grabham abt 1849 Cotleigh, Devon, Wife Married Charwoman
Annie Grabham abt 1866 Yarcombe, Devon, Daughter Farm Servant
Bessie Grabham abt 1872 Upottery, Devon, Daughter Scholar
Abraham J. Grabham abt 1880 Upottery, Devon, Son
Henry J. Ware abt 1872 Upottery, Devon, Step Son Scholar
On the 1871 census however this family's entry is
18871 census RG11 2040 folio 69 Woodcott Cottage Upottery Devon
Abraham Grabham 1843 Yarcombe, Devon, England Head Married Ag lab
Hannah Grabham 1843 Bradford, Somerset, Wife Married
William Grabham 1864 Yarcombe, Devon, Son
Bridget A Grabham 1866 Yarcombe, Devon, Daughter
Emma J Grabham 1869 Yarcombe, Devon, Daughter
Though I can find on FreeBMD Abraham's later marriage in 1879 in Honiton to Emily Ware I cannot find his previous marriage to Hannah. I cannot find in FreeBMD (an incomplete index) any marriage or death of a Bridget Grabham, so it is possible she dropped the use of Bridget after her mother's death and was known as Hannah/Ann(ie). The fact that she was living in Upottery in 1871 and 1881 might also be consistent with her giving this as her birth place on the 1901 census.
On the face of it since Eli Northam's wife Sarah Ann Nills was born in Yarcombe circa 1840 there seems no obvious connection to Hannah Grabham, the wife of Abraham born circa 1843 in Bradford Somerset.
Without knowing the name of Hannah Grabham's father on the 1896 marriage certificate it is impossible to say whether (Bridget) Annie is a potential candidate or not.
Regards
Valda