The 1900 census confirms that in June that year, they had been married for 7 years.

James can be seen in 1851 living with his wife at her parent's house at Westbury, Buckinghamshire, England:
James KNIBBS Married 22 Railway Labourer born Deddington
Ellen KNIBBS Married 19 born Westbury

Ellen was the daughter of Robert and Ann Collin
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The exact date of when they emigrated to the US isn't clear. The earliest we can seem James and his family living in the US is on 9 July 1860 when they were living in a rural area of Farmington Township, Hartford County, Connecticut. However, the 1900 census record against James daughter Anne, suggests that they arrived in the US in 1855.

The following is extracted from the 1860 census:
James Nibbs, age 33, male, occ. laborer, owns personal property worth $50, born in England.
Mary, age 32, female
Ann, 8, female
Eliza, 4, female
William, 3, male
Burritt, 2/12, male

The official census day in 1860 was 1 June. Burritt is later called Benjamin. I am not sure where the name Mary came from. His wife was Ellen - almost certainly a mistake made by the enumerator. James' personal estate was valued at �50
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James is listed in the Unionville part of the Farmington CT census for 1870:
KNIBBS JAMES, 42, Plaster Mason, England
KNIBBS Helen, 39, Keeping House, England
KNIBBS Ann, 18, England
KNIBBS Eliza, 15, England
KNIBBS Wm H., 13, Conn
KNIBBS Benj, 10, Conn
KNIBBS James, 6, Conn
KNIBBS Helen, 4, Conn
KNIBBS Joseph W., 1, Conn
value of real estate $1800.00, value personal $100.00
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James appears in the 1880 census for the United States, living at Farmington, Hartford County, Connecticut. The entire family are listed as KRIBBS instead of KNIBBS:
James KRIBBS 50
Ellen 49
William H 23
Benjamin 20
James 16
Hellen 14
Joseph W 11
John E 7
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We can see also James arriving in New York on 16 Feb 1894 aged 66, with his son Wesley aged 25. They had sailed from Liverpool via Queenstown. They were on the vessel The Majestic. No-one else seemed to be with them.

James seems to have picked up the middle initial of "D" in America.
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I am extremely lucky to have the above photograph. It was purchased in a job lot of 4,000 picture postcards at auction by a man in America. He contacted me and asked if I would like to buy it from him.
It was taken on 8th May 1908 and shows James KNIBBS aged 78 years. With him are his daughter, Mrs Root aged 56, her daughter Mrs John Habershon aged 34, her daughter Mrs Archie Young aged 17, and her son Archie Arthur Young aged 6 months.
I believe Mrs Root is his daughter Ann Knibbs having remarried after her first husband died in 1898. Mrs John Habershon is the right age to be Ann's daughter Minnie from Ann's first marriage to Elija Roberts.
(Click on the photo to enlarge).
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James can be seen on the 1900 census, born August 1826 in England, nationalized in 1857, and married for 7 years.
His wife Julia is also listed, born September 1830 in England. Julia is listed as having had 5 children, all of them alive at the time of the census.
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We can see that in 1910, James was living with his son and daughter-in-law Benjamin and Emma at Farmington, Ct.
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From the Hartford Courant, Jun 27, 1914:

DEATH OF JAMES KNIBBS,
VETERAN MASON-BUILDER

Unionville, June 26.

     In the death of James Knibbs, whose funeral was this afternoon, Unionville loses a man who during his fifty-five years residence in the village endeared himself by his geniality and unfailing friendship. Born in Clifton, Deddingtonshire, England, in 1829, at the age of twenty-one he married Miss Ellen Collins of his native city, who was then in her nineteenth year. Emigrating to this country in 1858 they settled in Unionville and the family home remained here ver since. Mr. Knibbs was a builder and mason until very recent years when his advanced age caused him to retire from active work and he busied himself about his farm in the West District. He delighted in remembering the hardships with which workmen were formerly confronted and comparing present conditions with the past. In olden times he was employed in West Hartford and masons at that time had to work ten hours for a day's pay. He walked the eleven miles to and from work each day and never thought of losing any time from his labor.
     His wife died in 1885 and Mr. Knibbs leaves seven children and descendants to the fifth generation. The children are William H. and James of Waterbury, Mrs. O. A. Root of Southington, a daughter, Mrs. Delmore Wetherby, and three sons, B. R., J. W., AND J. E., of Unionville.
     A picture which he prized highly showed him with the five generations of his family. They are Mr. Knibbs, Mrs. O. A. Root, his eldest daughter: his granddaughter, Mrs. John Habershon of South Meriden, Mrs. Irene Young of South Meriden, a great-granddaughter, and Mrs. Young's children. Mr. Knibbs was in his eighty-sixth year.
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James was buried at the Greenwood Cemetery, Avon, Hartford County, Connecticut

The 1900 census shows that Julia had 5 children, all of whom survived. It also shows that she emigrated to the US in 1888.
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We can see a record of Julia having travelled back to England at around 1900 or 1901. The record of her return journey is shown at the Ellis Island database. On 10 March 1901, she arrived at New York aboard the vessel 'Germanic'. It appears that she was travelling alone. Her end destination was identified as Unionville, Connecticut, where it states she would be joining her husband James.