Interpret this bible verse: The law that says you must have intercourse with your dead brothers wife?

This extends beyond actual brothers. Read the book of Ruth. Boaz was not Naomi's Brother in law much less the brother in law of Ruth yet the laws of Leverit marriage were understood to apply to them anyway. Also you should note that a person could refuse, in the book of Ruth there was a man who was more closely related than Boaz who refused to marry Ruth.