I still see this happening mainly to keep w/ traditions but enforcement is consensual. The reason behind is everybody wants to know the widow and her family is being taken care of. Even if she is not given to the husband's brother, a family representative will make a promise to look after her family. Sometimes they will say things like, "We know we can never replace our brother but we will make sure she and her family will not go hungry, has roof over their head, something something...et c.."