The 1851 census gives William Pickett's birth place as Merone in Devon, this place name is not found in the Devon Gazetteers,, The best fit for William is the baptism of William Peckett, son of William and Grace baptised on 11 Mar 1770 in Monkokehampton.
William Pickett circa 1770 in Devon, with Elizabeth, born Weymouth, had four children baptised in Millbrook, Southampton.
Louise b. 1807, Stephen b. 1809, George b. 1810 and Caroline 1812. A marriage not found.
Caroline Pickett b. 1812 is my 3 x Great Grandmother, she married firstly, George Boyce in Millbrook on 30 Jan 1830.
George died that same year in the April. They had one son George b. 18 Oct 1830, my 2 x Great Grandfather.
Caroline married secondly, William Curtis on 23 Dec 1832 in Millbrook.
They had a son William 1834 - 1836, Caroline died on 16 Oct 1836.
William then married Caroline's elder sister Louisa on 14 Jan 1838 in St Mary Portsea.
They had two children Joshua in 1841 and Caroline in 1843. Both bapt., in Millbrook.
William Curtis died in 1844 and Louisa in 1848, hence the children are with the grandparents on the 1851 census at Four Posts, Millbrook, Southampton
It was against the law to marry your dead wife's sister, but, when did that stop folk! There are such cases in my family, one well documented, as a court case followed, one on the Isle of Wight where the vicar baptised the subsequent children as base born.
I would think that is why the marriage was in Portsea. Interestingly there is a marriage in Portsea of William Peskett to Elizabeth Lane on 21 Jun 1804 ? Needs some more work.