Herbert's father, Henry Theodore Knibbs isn't strictly a member of the Knibbs family. His parents were John and Ann BURT so his real name is Henry Theodore BURT. Henry's mother was widowed and then married Francis Knibbs (1793-1856).
Henry married as Henry Theodore BURT but at a later date, he assumed the surname KNIBBS.
__________

We can see Herbert in 1891, aged 20 and unmarried, living in St George, London, Middlesex at the household of his father, Henry Knibbs. The household seems a little mixed and a struggle to understand who is who.
Henry Knibbs Head Widr 63 Beerseller Somersetshire
Sydney Knibbs Son M 26 Manager St George's East
Alice Knibbs Wife M 26 Middlesex
Alice Knibbs Dau 2 Bloomsbury, London
Grace Knibbs Dau 8 Bloomsbury, London
Lizzie Knibbs Wife M 56 Middlesex
Walter Knibbs Son 9 Middlesex
Alice M Dau 6 Bloomsbury, London
Eleanor Knibbs Dau 5 Bloomsbury, London
Frank G Knibbs Son 2 Bloomsbury, London
Herbert Knibbs Son 20 Homerton
__________

Herbert can be seen in 1901 living at Islington, London with his wife and two daughters:

Herbert Knibbs 30 Homerton London London Islington Parcel Sorter G P O
Eliza Knibbs 27 Bermondsey London London Islington
Doris Knibbs 2 Islington London London Islington
Ada Knibbs 1 Islington London London Islington
__________

We next see them living at Edmonton, Middlesex at the 1911 census:
Knibbs Herbert M 1871 40
Knibbs Eliza Burrows F 1874 37
Knibbs Lyall Gilda F 1897 14
Knibbs Doris Ethel F 1899 12
Knibbs Ada Edna F 1900 11
Knibbs Edith May F 1902 9
Knibbs Herbert Henry M 1904 7
Knibbs Walter Sidney M 1905 6
Knibbs Phylis Ida F 1907 4
Knibbs Leonard Reginald M 1910 1

Doris' grandfather, Henry Theodore Knibbs isn't strictly a member of the Knibbs family. His parents were John and Ann BURT so his real name is Henry Theodore BURT. Henry's mother was widowed and then married Francis Knibbs (1793-1856).
Henry married as Henry Theodore BURT but at a later date, he assumed the surname KNIBBS.
__________

Doris died aged just 13 years.

We know that Herbert was unmarried and spent some of his life in the merchant navy. In the early 1960's he worked on the P&O cruise ship, SS Arcadia, which operated on the UK-Australia passenger service.

__________

The following is an extract from the Times newspaper of 26 May, 1962:

IN THE ESTATE OF KNIBBS, DECEASED
FLAY v. TRUEMAN
Before Mr. Justice Wrangham
His Lordship holding that statements made by the deceased while he was a mariner at sea, that if anything happened to him his sister, Iris, should get anything he had, were not entitled to probate as nuncupative will, gave judgment for defendant, Mrs. Edith Mary Trueman, of Higham Road, Tottenham, in this probate action in which the plaintiff, Mrs. Iris Vera Flay. of London Road. Hadleigh, Essex, propounded an a privileged will of the deceased Mr. Herbert Henry Knibbs late of London Road, Hadleigh, a statement made by him on February 17, 1961 in the following terms: "If anything happens to me anything I have got is to go to my sister, Iris." The plaintiff propounded, in the alternative, a statement made in similar terms of April 7, 1960.
Mr. G. W. Cheyne appeared for the plaintiff; Mr. P. C M. Curtis-Bennett for the defendant

JUDGMENT
His Lordship said that the deceased was a seaman, and unmarried. He had a sister named Iris, to whom be was more attached than to any other member of his family, and whom he had named as his next-of-kin in the records of his employers. In 1960 the deceased was working as a barman In the liner Arcadia. In April 1960 his sister was a passenger in the Arcadia to Melbourne, where she disembarked. On the evening after the Arcadia left Melbourne, the deceased said to the head barman, Mr. Wills, in the course of a conversation on domestic matters,: " If anything happens to me Iris will get anything I have got." In February, 1961. when the Arcadia was on another voyage, the deceased had a similar conversation with another barman. Mr. McGee. The deceased died on March 1, 1961, at the age of 57, on shore, from coronary thrombosis. The question arose whether what the deceased had said to either or both of these men was a nuncupative will. The fundamental principle to be applied was that nothing could be admitted to probate which was not intended to be a testament A testamentary act for the purposes of a privileged will did not have to be a document or an act attended by any formalities. It was not even necessary for the speaker to know that he was making a will or that he was capable of making a will when he uttered the words in question. In order to constitute a testamentary act, however, there must be a statement of the deceased’s desires for the disposition of his property which was not merely imparted to his audience as a manor of information but was intended to convey to that audience a request to see that those wishes were acted upon.
His Lordship was satisfied that the deceased had formed a clear and definite intention that his sister, Iris, should have all his property at his death. If it were possible to give effect to that intention, it would be just to do so. But the Court had to administer the law as it was. His Lordship did not think that when the deceased had this not much more than carnal conversation in the bar with Mr. Wills, he was intending to perform a testamentary act. He was merely discussing family matters. If the deceased had told Mr. Wills that he wanted his sister Iris to have all his property after his death, and had asked Mr. Wills to see that she did have it, the situation might have been different. His Lordship was not satisfied that anything had been proved with sufficient clarity in regard to the later conversation with Mr. McGee. In the result, neither of the statements propounded could be admitted to probate, and they would be pronounced against. There would be judgment for the defendant, with costs out of the estate.

I understand from Leonard's nephew, that Leonard suffered a head injury aboard ship whilst serving in the Royal Navy. In later life, he became a tramp and was last seen in about 1961.