Hi Brian,
I can neither confirm nor deny because I don't have access to Parish Registers, but I have taken a look at the IGI for Whitton using the Hugh Wallis site and I see baptisms there are covered up to 1812 and the only Cousins listed are children of Thomas & Ann, all born between 1783-1792. Included in this list are a Sarah and an Ann and I would suggest that if your Sarah Ann was also a daughter then both Sarah and Ann probably died as infants and their joint names were given to her, although I would have expected to find her baptism along with the others.
Baptisms at Whitton, Lincs:
James Shankster Cousins - 28 January 1783. Parents: Thos. Cousins & Ann.
Isaac Cousins - 15 August 1784. Parents: Thomas Cousins & Ann.
Sarah Cousins - 28 April 1786. Parents: Thos. Cousins & Ann
Ann Cousins - 21 May 1787. Parents: Thomas Cousins & Ann.
Margaret Cousins - 5 February 1790. Parents: Thomas Cousins & Ann.
Mary Cousins - 21 July 1792. Parents: Thomas Cousins & Ann
I also looked at marriages:
Marriages at Whitton, Lincs:
Ann Cousins = John Westoby on 6 December 1803.
Sarah Cousins = James Pickersgill on 9 May 1803.
Jane Cousins = James Stephenson on 16 November 1795.
Ann Cousins = Anthony James on 17 February 1789.
Robt. Couzons = Cath Clarvis on 15 May 1759.
My thoughts may not be of any use to you whatever, but nevertheless I shall give them! All the above people would appear to be members of the same family. There are four marriages of girls indicating that Thomas probably married out of the parish. The one male marriage i.e. Robert could have been their father. You say Sarah Ann was born 'About' 1804. If this date is just a couple of years out it is not beyond the realms of possibility that she was an unbaptised illegitimate daughter of one of the four girls.
Sorry I can't be of more help!
Jill