Sarah Chettleburgh was the unmarried mother of William Chettleburgh.
William's uncle was also called William, and he lived with her son William after he retired (according to the 1881 and 1891 censuses). If the hypothesis that Sarah was a Chettleburgh by birth is true, then this would make William the elder her brother (rather than brother-in-law).
There is a record of a Sarah Chettleburgh (spouse unknown) christening a William Chettleburgh on the 3rd December 1837 in Saint John De Sepulchre.
Sarah was born in Lakenham on the 6th December 1807 to Robert Chettleburgh and Elizabeth Burgoine according to the Lakenham parish registers. She was the second Sarah born to those parents - the first was born on the 10th August 1806, but died the next month (30th September 1806).
She had a brother, Robert Burgoyne Chettleburgh, born on the 5th June 1809 (in Lakenham).
The same parents also had a son (our missing Uncle) called William on the 22nd March 1812, again in Lakenham. And again this was their second William, the first was born on the 24th March 1811.
Other (unrelated?) reports:
There is a record of a Sarah Chettleburgh Christened on 9th November 1823, in Saint Clement, Norwich, Norfolk. However this is probably a different Sarah as 14 is a bit young for having a child.
There is a Sarah Chettleburgh in Spitalfields, Thorpe Next Norwich in 1851 (she was 33 and born in Catton, Norwich, Norfolk) who is an unmarried Dress Maker, the head of the household with a daughter called Esther A. Chettleburgh, and a nephew called James (both children born in Thorpe Hamlet, Norfolk).
A William Chittleburgh married a Frances Cooper on the 4th November 1806 in Topcroft, Norfolk (IGI). Now we have records (1851 census) of another Frances Chittle
A Sarah Chittleburgh was born in Woodton in 20th March 1831 to John and Elizabeth Chittleburgh.