TSK · Ruth 3:2

If brethren dwell together, and one of them die, and have no son, the wife of the dead shall not be married without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of a husband's brother unto her.

Then said Boaz unto Ruth, Hearest thou not, my daughter? Go not to glean in another field, neither pass from hence, but abide here fast by my maidens.

For both he that sanctifieth and they that are sanctified are all of one: for which cause he is not ashamed to call them brethren,