That seems overly restrictive. Consider this scenario: 1. Romeo and Juliet meet when they are teens. They are not step-relatives. They develop a romantic relat

That seems overly restrictive. Consider this scenario: 1. Romeo and Juliet meet when they are teens. They are not step-relatives. They develop a romantic relationship, which their parents disapprove of. 2. The maintain their relationship, which grows deeper with age, and plan to marry as soon as the are 18. 3. Before they turn 18 both of them lose a parent (maybe to divorce, maybe to death). Their remaining parents get married in order to make Romeo and Juliet step siblings and thus block them from marrying.