George was living at Old Windsor Green, Old Windsor, Berkshire, England in 1881 with his wife and two children:
George Knibbs Head 39 Ostler Old Windsor, Berkshire
Ann Maria Knibbs Wife 25 Ostlers Wife Egham, Surrey
William Knibbs Son 4 Old Windsor, Berkshire
Ann Knibbs Daur 2 Old Windsor, Berkshire
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George died age 46 years.

We see George's widow in 1891 living at Egham, Surrey:

Ann Knibbs Head 35 Female Egham, Surrey
Emily Knibbs Daughter 16 Female Egham, Surrey
William R Knibbs Son 14 Male Egham, Surrey
Annie Knibbs Daughter 12 Female Egham, Surrey
Charlie Knibbs Daughter 7 Female Egham, Surrey
John Knibbs Son 4 Male Egham, Surrey
William Bryant Boarder 13 Male Egham, Surrey

Town: Egham, Ecc. Parish: St John the Baptist, Civil Parish: Egham, County: Surrey
Registration: Windsor, Egham, 11
Source: PRO/National Archives Reference # RG12/1010 Folio 36 Page 33
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In 1892, Ann married Charles Thomas Mansfield. Sadly he died less than a year later, and it seems that Ann reverted back to using the surname Knibbs.
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In 1901, we see Ann, now twice widowed, living at Egham, Surrey, but am not sure why she reverted to using the sruname Knibbs after her 2nd marriage:

Ann Knibbs Head Widow 44 Egham Surrey
George Knibbs Son 17 Groom Domestic Old Windsor Berks
John Knibbs Son 14 General Servant Old Windsor Berks
Louisa Knibbs Daughter 6 Egham Surrey
Edith Knibbs Niece 12 Old Windsor Berks
George Howard Visitor (Single) 46 Carter Egham Surrey

Edith is the daughter of Ann's brother-in-law Edward Knibbs and his wife Frances. Edward died in 1900 and Frances in 1893 leaving Edith an orphan at the age of just ten years of age.
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We see Ann again in 1911 living at Chester Cottages, Blays Lane, Windsor, Berkshire:

Ann Knibbs, Head Widow F 60 Laundry Worker Egham Surrey
Emily Knibbs Daughter Single F 36 Laundry Worker Old Windsor Berks
William Knibbs Son Single M 34 Plumber Old Windsor Berks
Louisa Knibbs Daughter Single F 17 Englefield Green Surrey
Albert Knibbs Grandson M 11 Scholar Englefield Green
Agnes Knibbs Granddaughter F 5 Portsmouth Hants
George Howard Boarder M 61 Farm Labourer Englefield Green Surrey

I believe that Louisa is Emily's daughter, Albert is Emily's son, and Agnes is a daughter of Amos Knibbs who is Ann's nephew, living in Portsmouth.

Was it perhaps this William Knibbs who was in a spot of bother as a 16 year old in 1893 which, on the face of it, seemed to be a prank that got out of hand. The case was heard at the Old Bailey, London's Central Criminal Court on 11 Dec 1893:

139. CHARLES WINKWORTH, GEORGE HAINS, GEORGE TRUMPER, WILLIAM KNIBBS, ALBERT HESSEY, ARTHUR ADAMS , and ERNEST WELLBELOVE, Maliciously and feloniously setting fire to certain furze and heath growing on Englefield Green.

MR. G. K. T. PURCELL Prosecuted.

RICHARD EDWARD WOOTTON (39, Surrey Constabulary). At nine p. m. on August 30th, I was on duty at Egham Hill, and in consequence of seeing a light proceeding from a fire, I went to Englefield Green and saw the furze and heath on fire - there was a great fire there - I saw all the prisoners (except Wellbelove, who joined them afterwards) taking the burning furze and setting fire to other furze - I knew the prisoners before - when they saw me they shifted away to different parts and then came back again and set fire to other parts while I was there - I have not the slightest doubt all the prisoners, except Wellbelove, are the boys I saw doing this - Wellbelove was not there at a quarter past nine - I did not see him.

Cross-examined by Hessey. You were taking part in it.

By the COURT. There were fifty people there, I daresay, in different parts, youngsters, and people who were trying to beat it out - I saw it alight at nine o'clock, and I was half-a-mile away then.

WILLIAM HART . I am a butler in the employment of Mr. Raphael, of Castle Hill, Englefield Green - on the night of this fire I went outside the house about eleven p. m. - I saw the fire, and I saw a lot of boys there, about twenty or thirty, I suppose - I saw Wellbelove standing there with the rest of them, just kicking the fire about - I did not see him setting fire to anything - he gave the names of the others after I handed him over.

The JURY intimated that It was waste of time to proceed further with the case, as there was no evidence of how the fire originated.

NOT GUILTY
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I understand it may have been this William Knibbs who was killed in action diring WWI:

In Memory of Private WILLIAM KNIBBS
S/48, 6th Bn., The Buffs (East Kent Regiment) who died on 04 August 1916
Remembered with honour THIEPVAL MEMORIAL

His name is inscribed on the war Memorial at Epsom, Surrey, England, alongside that of his brother George Charles Knibbs, also killed in action during WWI.

In Memory of
Private G C KNIBBS

28808, 5th Bn., Royal Berkshire Regiment
who died age 37 on 24 July 1917
Son of Mrs. A. Knibbs, of 30, Middle Hill, Englefield Green, Surrey.
Remembered with honour
MONCHY BRITISH CEMETERY, MONCHY-LE-PREUX

His name is inscribed on the war Memorial at Epsom, Surrey, England, alongside that of his brother William Knibbs, also killed in action during WWI.