The online Death Duty Registers' index only goes up to 1811
http://www.nationalarchives.gov.uk/documentsonline/death-duty.aspThe Death Duty registers index is at The National Archives and the Family Records Centre and the Death Duty Registers have been microfilmed up to 1858 and are found at both places. Post 1858 the large original registers are stored at Hayes and you have to give 3 days notice to view them at The National Archives.
I don't know for sure whether Thomas Lewin of Swineshead is Jonathan senior's brother and or whether his will is going to supply you with any further useful information even if he is (there is always a chance any man's will just states everything left to his wife and children - and not even names them) however
1851 census HO107 1750 folio 617
Swineshead Huntingdonshire
James Lewin 54 Ansty, Leicestershire, Head Married Farmer of 400 acres employing 12 labourers
Elizabeth Lewin 48 Swineshead, Huntingdonshire, Wife Married
John Lewin 23 Swineshead, Huntingdonshire, Son
Joseph Lewin 17 Swineshead, Huntingdonshire, Son
Jonathan Lewin 15 Swineshead, Huntingdonshire, Son
Elizabeth Lewin 13 Swineshead, Huntingdonshire, Daughter
James Lewin 10 Swineshead, Huntingdonshire, Son
Eliza Lewin 5 Swineshead, Huntingdonshire, Daughter
plus 2 servants
IGI Church member submitted record
James Lewin
Christening: 12 JUN 1796 Anstey, Leicester
Father: Thomas Lewin Family
Mother: Elizabeth Atkins
Thomas Lewin
Christening: 19 AUG 1787 Anstey, Leicester
Father: Thomas Lewin
Mother: Elizabeth Atkins
There are probably other siblings but I just checked for James and a brother Thomas since Jonathan mentions his nephew Thomas.
1851 census HO107 1468 folio 846
Anstey Villa Pembridge Villas Kensington Middlesex
Thomas Lewin 63 Ansty, Leicestershire, Head Married Woollen Draper
Mary Lewin 59 Dunchideock, Devon, Wife
Mary A Lewin 20 Westr, Middlesex, Daughter
Martha Lewin 19 Westr, Middlesex, Daughter
plus 1 servant
Regards
Valda