Mary Ann listed as a widow of Deddington parish. The parish of Francis is not identified.

Mary Ann signed the register as Killby with two "l's".

Listed in the 1851 Census of the Woodstock Union as living at Philcock Street, Deddington with his wife Mary Ann, son John Francis and daughter Lydia.

Deddington, Philcock Street:
Francis KNIBBS 35 Road Labourer Deddington
Mary Ann 32 Deddington
John Francis 4 Deddington
Lydia 1m Deddington

This is the first I've ever seen of a daughter Lydia. No sign of her baptism in Deddington
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Francis can be seen in 1861 living at Buckingham, Buckinghamshire, England:
Francis KNIBBS 1816 Deddington Oxfordshire Head Buckingham Buckinghamshire
Mary Ann KNIBBS 1819 Deddington Oxfordshire Wife Buckingham Buckinghamshire
John J KNIBBS 1847 Deddington Oxfordshire Son Buckingham Buckinghamshire
Lydia KNIBBS 1851 Deddington Oxfordshire Daughter Buckingham Buckinghamshire
Emma KNIBBS 1858 Deddington Oxfordshire Daughter Buckingham Buckinghamshire
Elizabeth MALINS 1827 Deddington, Oxfordshire Visitor Buckingham Buckinghamshire

Francis is believed to have been running the Kings Arms public House, West Street, Buckingham.
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The 1881 census shows Francis' wife Mary as being widowed, so clearly, Francis died before 1881 and quite probably before 1871 as he doesn't appear then either.

Mary's father was Thomas Sykes, a labourer.
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Mary appears in 1871 living at Buckingham, Buckinghamshire, England with her daughter Emma:
Mary Ann Knibbs abt 1821 Diddington Oxfordshire Head Buckingham Buckinghamshire
Emma Knibbs abt 1859 Diddington Oxfordshire Daughter Buckingham Buckinghamshire

Her husband Francis wasn't with her, so it is presumed that he died before 1871.
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Mary appears in the 1881 census as Mary A Knibbs, widowed, living at Bone Hill, Buckingham, Buckinghamshire, England
Mary A. KNIBBS Lodger (Head) W Female 60 Deddington, Oxford, England Charwoman

I see in the census that living next door to Mary there was a man named Joseph North, aged 73, a widower with the trade of Hay Tier, although he's identified in the census as "unable to work". I wonder, was he the owner of the house she was lodging in, and was she perhaps looking after him? - was the William North she married his brother? All pure speculation on my part.
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The Bucks Herald on 26 August 1882 reported a marriage at Buckingham on 20th August 1882 between William North and Mrs Emma Knibbs. The GRO has this marriage listed for Mary Ann Knibbs. I believe the GRO would be the more accurate source of information, so I presume this was a third marriage for Mary Ann Sykes.
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It's interesting to note also that Mary's son John Francis Knibbs married a girl named Cecelia North - any relation to Mary's third husband perhaps?
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There's no sign of Mary living at Buckingham in the 1901 census, but there's a William North, aged 82, occupation Hay Tier, so I wonder if this William North could be Mary's widowed husband?