Also - do you have the death certificate please, for the one who died in 1840? The person who registered the death may be one of her children - then you would know.
In addition - you say you have the burial details - was the burial at the same place as her husband?
(Sorry about all the questions.)
The 1861 Sarah Ventham - I can't see in 1841 - in fact I cannot immediately see ANY Sarah Ventham in Hampshire in 1841 -- but we know that one of them died. The other Sarah may have married or remarried to someone called Ventham between 1841 and 1861. (Unless of course, someone has found a Sarah Ventham in 1841 of the right age - rounded down.)
Rosie - that baptism date is the same date as that one posted by sjjs.