States regulate marriage between blood relatives, and incest is a crime in all 50 states. Marriage between two people who are closely related by blood is likewise illegal, but step-siblings aren't blood relatives. Legally, they are considered related only because their parents got married. Typically, if that parental marriage bond dissolves, the step-relationship is also dissolved. In practice, step-siblings can remain close even after their parents split up.
It doesn't strain logic that step-siblings could fall in love with each other. This would potentially be more likely if the couple already knew each other before their parents got together. As long as they don't live in Virginia, no part of their relationship is illegal. In all but one state, step-siblings are legally free to get married. Virginia takes a different approach, and step-siblings cannot marry in the state even if their parents are no longer together. Under Virginia law, it makes no difference how long the parents have been divorced or even whether the couple ever lived together as step-siblings.
It is illegal for step-siblings to marry in Virginia, even if the step-relationship was through a previous relationship that ended.
All states prohibit marriage between people who are closely related by blood. In many states, engaging in an illegal marriage could result in a felony conviction and prison time. Some states also prohibit marriage between a stepparent and a stepchild. Arkansas, for example, prohibits marriage between a stepparent and a stepchild, as well as a stepgrandchild. Only Virginia prohibits marriage between step-siblings.
Some of these laws have odd conditions. For example, Alabama prohibits marriage between a stepparent and a stepchild, while the marriage that created the step-relationship still exists. Since the stepparent would have to divorce before remarrying, this law seems redundant. These marriages are prohibited under criminal incest statutes. Thus, you have states defining a step-relationship as incestuous, at least in regard to the stepparent/stepchild relationship.
The Incest Question
Incest is a near-universal taboo. Marriage or sexual relations between two people who are closely related by blood is frowned upon in almost all cultures globally. An incestuous relationship between a parent and a child is perhaps the most abhorrent. Incestuous relationships between siblings, while illegal, may not raise many of the same objections. For example, in 2014, a majority in the German Ethics Council voted that consensual sexual relations between adult siblings should no longer be a crime. In the U.S., incest is a crime in all 50 states. In many, incest is a felony offense that could lead to 10 or more years in prison. However, in some states an incestuous relationship is not a crime in and of itself as long as the couple doesn't attempt to marry. For example, in New Jersey only incestuous marriages are illegal, but incestuous relationships are not.
Most people provide practical, objective reasons for the prohibition against incest, such as the increased risk of birth defects among the children of parents closely related by blood. However, the real reason incest is rejected socially and culturally seems to be a moral issue, rather than a practical one. Indeed, the German Ethics Council noted that social taboos would remain even if the legal prohibition was removed.
A Moral Dilemma
Laws that treat sexual or marital relationships between stepparents and stepchildren as illegal incest ignore the objective, health-related reasons to prohibit sexual relations between close relatives. Stepparents and stepchildren may not be related by blood, but they have a relationship that begins as a parent/child relationship. This is a relationship most people hold sacred. A stepparent having sexual relations, or seeking to marry, a stepchild seems just as offensive as a father having sexual relations with his daughter, or a mother with her son.
However, to make those relationships a crime may seem to some like an attempt to legislate morality. This was the conclusion of the German Ethics Council, which found that the risk of genetic impairment to any children was insufficient to justify the complete ban on relationships between siblings. The Virginia law banning marriage between step-siblings places them in the same category as siblings.