My friend was always aware that his grandfather had been married twice, and that the first wife died. His grandmother was the second wife, and his DF was the only child of that second marriage. There were some children from the first marriage.
Some family history research has revealed that his grandfather’s first and second wives were sisters. Two years after the first wife died, the widower married his sister-in-law. The law allowing this to happen was only passed in 1907, about 20 years earlier.
My friend is not particularly interested in family history. But I was really taken aback when he said marrying a dead wife’s sister is probably more common now than it used to be! 
I completely disagree - I think very few women would have any interest at all in marrying their brother-in-law if their sister died.
He thinks I’m wrong. I said I’d put it to MN.
AIBU?