John Gill Commentary Mark 12:20

"There were seven brethren: and the first took a wife, and dying left no seed;" — Mark 12:20 (ASV)

Now there were seven brethren
In a certain family, at a certain place; perhaps at Jerusalem, who were brethren by the father's side; for such only were reckoned so, and such only did this law oblige:

and the first took a wife, and dying left no seed :
no child: hence it is clear that the ancient Jews used the word seed, of a single person, as these Sadducees did; though modern ones deny such an use of it in our present controversies with them about the sense of (Genesis 3:15) (22:18) ; (See Gill on Matthew 22:25).