John Gill Commentary Mark 12:19

"Teacher, Moses wrote unto us, If a man`s brother die, and leave a wife behind him, and leave no child, that his brother should take his wife, and raise up seed unto his brother." — Mark 12:19 (ASV)

Master, Moses wrote to us
Has left in writing for us the following precept to observe; for they acknowledged the writings of Moses, and indeed all the Scriptures of the Old Testament; adhering to the literal sense of them, and rejecting the traditional interpretation of them by the Rabbins:

if a man's brother die, and leave no children, that his brother
should take his wife, and raise up seed to his brother ;
which is the sense of the law in (Deuteronomy 25:5) ; (See Gill on Matthew 22:24).