Oh dear, Middlesex is not very useful as a place of birth. : (I assume Stephen was dead by the time of the1861 census or you would have checked place of birth stated then.)
However, you are lucky in that you are looking for quite an unusual surname. I found this on the Family Search site
STEPHEN PEET
Male
Event(s):
Birth: 25 APR 1819
Christening: 17 OCT 1819 Old Church, Saint Pancras, London, England
Parents:
Father: EDWARD PEET Family
Mother: MARY
Assuming Stephen was married to Phoebe (and after 1 July 1837) do you have his marriage certificate? This should give his father's name and at least indicate whether the St Pancras christening is a possible. The names of the witnesses may give you further clues as to siblings.
Barbara