Thanks for the various contributions on who you can and can't marry. It seems that in England in the old days that you couldn't marry your deceased wife's niece, but I have an idea, picked up from somewhere that the rules were not so clear in the IOM, where the church seems not always to have towed the party line, and this impression tends to tie up with Nigel's comment about the marriage being voidable.
From my point of view, there are the two issues, (a) Was a deceased wife' niece OK", but also the conundrum as to whther Jane Knipe (maiden name Wattleworth) was Archdeacon Sam Wattleworth's daughter or not. Wm Casement's second wife was clearly the daughter of Sam's daughter and an Irish Clergyman, Thomason, but definite identification of Jane's parents remains obscure.
We may therefore have a deceased wife's niece situation or they may be distant cousins. At present the jury is out on that one !