In some societies, if a husband dies and leaves a wife and children, his widow is expected to practice the custom called the levirate. This means that she is expected to.

she is expected to marry her husband's brother

Explanation:

Levirate  is derived from the latin word "levir" which means husband's brother. Levirate marriage is a  marriage in which a woman is compelled to  marry the brother of  her  deceased husband . This type of marriage is usually practiced  in patriarchal societies that do not allow marriage outside one's social group or clan and most of these societies practice such to continue the line of the family especially if the widow has not borne any child with the deceased husband, or to give the new widow  and her children protection and security.

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