I was surprised to see " Until 1912 it was illegal in Britain for a widow or widower to marry their dead spouse's sister or brother (ref .Leviticus chapter 18, verses 14 to 18). "

I have looked this up.

I would have taken v16 to refer to "thy brother's wife" while he is still alive, although for inbreeding purposes I can see why they would avoid his widow.  There was an old belief called 'bad blood' which supposed that a widow's offspring were affected by her first husband (even after 9 months elapsed).

The counter-examples are Deuteronomy chapter 25, verse 5.  (If brothers are living together and one of them dies without a son, his widow must not marry outside the family. Her husband's brother shall take her and marry her and fulfill the duty of a brother-in-law to her.)

and Matthew chapter 22, verse 24

"Teacher," they said, "Moses told us that if a man dies without having children, his brother must marry the widow and have children for him."  

The Old English Law is set out in the back of the Book of Common Prayer 1662.

'A Table of Kindred and Affinity' wherein whosoever are related are forbidden in Scripture and our Laws to marry together.  This includes many relations with no genetic links.

The reason I was surprised was that Onan's sin in Genesis 38, 4-10 is allegedly not the 'spilling of his seed on the ground', but failing to impregnate his brother's widow. This suggests it was compulsory.  (I don't know where this leaves Onanism!)

"We used to have a budgie called Onan.  He always used to spill his seed on the ground"

Eric Drummond