Yes. It is illegal to marry your brother or sister in every state. No state issues marriage licenses to siblings, and no state recognizes such a marriage as valid. Depending on where you live, trying to do so can also lead to criminal charges. The details vary from state to state, particularly for half-siblings, adopted siblings, and step-siblings, so it helps to understand how the laws work.
Marriage Between Siblings Is Prohibited Nationwide
There is no federal law that sets the rules for who can marry. Each state decides that itself, and every state has laws, often called consanguinity laws, that bar marriage between close blood relatives. Marriage between a brother and sister is illegal in all 50 states and the District of Columbia, and statutes set the degrees of blood relationship that are too close for marriage. Claims online that some state allows sibling marriage under special circumstances are wrong. If you see one, check the state’s own statute.
What Counts as a “Brother” or “Sister” Under These Laws?
Siblings are not all treated identically, so the details matter.
- Full siblings: Prohibited in every state.
- Half-siblings: Also prohibited. Incest statutes generally do not distinguish between full and half blood, and virtually all courts hold that the prohibited degrees include half-siblings. Sharing one parent is enough.
- Siblings separated by adoption: Being raised apart doesn’t change the law. In State v. Sharon H., half-siblings who were separated in childhood when one was adopted were charged with a prohibited marriage. A court held that an adoption statute ending legal ties to natural relatives does not remove the blood relationship for purposes of marriage prohibitions.
- Adopted siblings with no blood relation: This varies. Statutes barring adopted siblings from marrying have been challenged and held unconstitutional, and most courts, though not all, allow marriage between people related only by adoption. Some states, though, extend their prohibitions to adoptive relatives, so check the state’s statute.
- Step-siblings: These have no blood relationship, and most states do not prohibit them from marrying, though a few extend their rules to relationships created by marriage. Local law controls.
What Happens If Siblings Marry Anyway?
The marriage is void. In most states, a marriage between close relatives is treated as void from the start, meaning it never had legal effect. The partners would not receive the legal rights of spouses, such as inheritance, spousal benefits, or the ability to make medical decisions for one another. A marriage entered in another state that doesn’t allow it generally won’t be recognized in the state where the couple lives.
Criminal charges are possible. Incest and prohibited-marriage statutes exist in every state in some form, but they are not identical. Every state regulates incest in some way, particularly incestuous marriage, but not every state broadly criminalizes all consensual sexual relationships between adult relatives, and there is no general federal incest statute. Where the offense is charged, it can be a felony, and consent between adults is often not a defense. Applying for a license with false information about your relationship can also lead to separate charges such as perjury or fraud.
How Does This Compare With Cousin Marriage?
Cousin marriage is treated differently, which is a common source of confusion. First-cousin marriage is prohibited in about 24 states and allowed in the rest. Siblings, by contrast, are barred everywhere. If someone says “marrying a relative is legal in some states,” they are usually thinking of cousins, not siblings.
Why Do These Laws Exist?
States commonly point to several reasons: the genetic risks of children born to close relatives, the protection of family structures and children from abuse and coercion, and long-standing social norms. Courts have generally upheld these laws, and there is no serious legal movement to repeal sibling marriage bans.
If You Are Unsure About Your Situation
Questions about adoption, step-families, or unclear family histories can be complicated. If you are unsure whether a relationship falls within a state’s prohibited degrees, a family law attorney can review the statute and your facts. This matters most before applying for a marriage license, since false statements on an application can create separate legal problems.
The Bottom Line
You cannot legally marry your brother or sister in the United States. Every state prohibits it, such marriages are void, and criminal charges are possible in many states. The rules for adopted siblings, step-siblings, and cousins differ, so the specific state statute is what counts.
