Maria Knibbs is Thomas Carr's second wife. His first wife was Maria Oman who was in fact the aunt of Maria Knibbs.
So before he actually married Maria Knibbs, she was his neice.
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From the Daily News (London, England), Tuesday, October 16, 1877;

CARR-KNIBBS - October 10, at Southfleet Church, Kent, Captain T. Carr, of King's Lynn, Norfolk, late of Gravesend, Kent, to Maria, daughter of E.G. Knibbs, Esq., of Northfleet, Kent

Thomas Carr can be seen in 1861 as a visitor to the home of Maria's parents. Maria (layer to be his 1st wife) was also there at the time in the name of Maria Rushworth from her 1st marriage.
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Thomas Carr can be seen in the 1881 census living with his second wife Maria (Knibbs) Carr and 5 children. They were all living at 265 Strand, London, Middlesex, England.
Thomas CARR Head M Male 67 Lynn, Norfolk, England Licensed Victualler
Maria CARR Wife M Female 35 (In Doubt As To Place) Licensed Victualler (Inn Sert)
Thomas CARR Son U Male 19 Aldgate, Middlesex, England Inn Serv
Mary Ann CARR Daur U Female 18 Aldgate, Middlesex, England Inn Serv
William O. CARR Son U Male 17 East Grinstead, Sussex, England Inn Serv
Edith E. CARR Daur U Female 2 Strand, Middlesex, England
Lionel CARR Son U Male > 1 Strand, Middlesex, England
Kate HORTON Serv U Female 21 Newington, Middlesex, England Servant (Domestic)
Eliza ENTICOTT Serv U Female 18 Axminster, Devon, England Servant (Domestic)

This Maria is Thomas' second wife. His first wife was also named Maria. She was Maria Oman, the sister of Elizabeth Oman who married Edwin George Knibbs.

Thomas' second wife Maria was in fact his niece. She was the daughter of Edwin George Knibbs and Elizabeth Oman.

The children Thomas, Mary Ann and William are Thomas' children from his first marriage to Maria Oman, so they are in fact both first cousin and step-children of Maria, his second wife above.

Maria appears as a widow in the 1891 census, Living on Own Means at 2 Ernest Villas, Old Dover Road, Gravesend, Kent:
Maria Carr Head Widow Living on Own Means London
Edith E Carr Daur 12 Scholar London St Clements Danes
Lionel Carr Son 11 Scholar London St Clements Danes
Winifred Carr Daur 9 Scholar London St Clements Danes
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Maria appears in the 1901 census living at Gravesend, Kent, England with two children, Lionel and Winifred. :
Maria Carr 54 London Bethnal Green Kent Gravesend Living On Own Means
Lionel Carr 21 St Clements Dancs Kent Gravesend Joiner Carp
Winifred Carr 19 Strand London Kent Gravesend