Laws on marrying a family member vary by state, but the outer limits are the same everywhere: no state in the U.S. lets you marry a parent, grandparent, child, grandchild, or sibling, including a half-sibling. Most states also bar aunt-uncle marriages with a niece or nephew. The rules diverge from there, especially on first cousins, adopted relatives, and step-relatives. Federal law does not set who can marry; it defers to the state where the ceremony took place.
Relationships No State Allows
Direct-line marriages are prohibited in all 50 states. That covers parent and child, grandparent and grandchild, great-grandparent and great-grandchild, and so on, in both directions. Sibling marriages are equally off-limits everywhere, and it makes no difference whether the siblings share both parents or only one.
These bans are absolute. Consent, genetic counseling, and advanced age do not change the outcome. Most states extend the prohibition to an aunt or uncle marrying a niece or nephew.
How States Measure Family Closeness
State marriage statutes rely on consanguinity, meaning blood relationship, to draw the line. The most common method counts “degrees” by tracing a path up the family tree to the nearest shared ancestor and back down to the other person. Each step is one degree.
Under this counting method, siblings are second-degree relatives (one step up to the shared parent, one step down). An aunt or uncle and a niece or nephew are third-degree relatives. First cousins sit at the fourth degree. A statute that prohibits marriage “within the third degree of consanguinity” bans sibling and aunt-uncle unions but leaves first cousins open.
Some states also consider affinity, which is a relationship created by marriage rather than blood. Where affinity matters, marrying a former in-law such as an ex-spouse’s parent may be restricted. Affinity rules are less common and less uniform than blood-based ones.
First Cousin Marriage by State
First cousin marriage is where state laws split most sharply. Roughly a third of states allow it outright, including California, New York, Colorado, Florida, and Virginia. About half of all states ban it completely. The rest permit it only under specific conditions.
Those conditional states usually require that the couple be past reproductive age or complete genetic counseling before getting a license. Arizona allows first cousins to marry only if at least one cannot reproduce. Maine permits it after genetic counseling. Illinois, Indiana, and Utah allow it once both parties reach an age, often 50 or 65, at which having children is unlikely. The compromise reflects the genetic concern behind the ban: once children are off the table, the objection fades.
Step-Relatives, Adopted Relatives, and In-Laws
Step-Siblings and Step-Parents
Step-siblings share no blood, and no state bans their marriage outright. If your parent married someone whose child you grew up alongside, the two of you are legally free to marry anywhere.
Step-parents and step-children are treated differently. A handful of states prohibit those marriages under affinity rules, at least while the marriage that created the step-relationship still exists. Once that underlying marriage ends through divorce or death, even those limited restrictions usually disappear.
Adopted Relatives
Adoption creates a legally recognized parent-child or sibling relationship, and most states treat that bond exactly like a biological one for marriage purposes. If you were adopted into a family, you generally cannot marry your adoptive parents, adoptive siblings, or other close adoptive relatives, even though you share no genetic connection.
A few states carve out narrow exceptions, particularly for first cousins related only through adoption. Oregon prohibits marriage between adopted relatives within the degree of first cousin but exempts first cousins whose only connection is through adoption. Exceptions like that are unusual. Most people in this situation will find the prohibition applies.
What Happens If a Prohibited Marriage Occurs
A prohibited marriage does not simply carry a fine and continue. In most states, it is void from the start; legally, it never existed. No court order is needed to end it, though people often seek a court declaration to update records or resolve property disputes.
A smaller number of states treat certain prohibited marriages as voidable, meaning the marriage stands until someone successfully challenges it in court. The distinction matters. A void marriage produces no legal rights or obligations between the parties from day one. A voidable marriage is treated as valid for the period before a court strikes it down. Children born during either type of union retain their legal rights in virtually every state; the invalidity of the parents’ marriage does not make the children illegitimate.
Property After a Void Marriage
Dividing property acquired together during a void marriage gets complicated. Some states apply their normal equitable distribution rules anyway, so neither party leaves empty-handed. Others recognize the putative spouse doctrine, which protects someone who entered the marriage genuinely believing it was legal. A putative spouse can claim the same property rights as a legal spouse, including a share of jointly acquired assets, if they acted in good faith. Not every state recognizes the doctrine, and it only helps the party who did not know.
Criminal Penalties
Marrying a prohibited relative is a crime in nearly every state. Incest statutes typically classify the offense as a felony. Sentences vary enormously depending on the state and the specific relationship, ranging from roughly one to two years on the low end to 20 years in states with the harshest sentencing. Fines commonly run from several thousand dollars into the tens of thousands.
A few states handle adults differently. New Jersey does not criminalize incest between adults 18 and older, and Rhode Island does not criminalize it for people 16 and older, though both states still prohibit the marriages themselves. California requires a person convicted of incest to register as a sex offender, a consequence that follows the person long after any prison sentence ends. Military service members face an additional layer, because the Uniform Code of Military Justice can prosecute incest as a service-discrediting offense under its general article even when civilian authorities do not.
Getting Married in Another State
The general rule in American conflict-of-laws doctrine is that a marriage valid where it was performed is valid everywhere. The rule has a well-established exception: a state can refuse to recognize an out-of-state marriage that violates its own strong public policy, and marriages between close relatives are the textbook example. If you travel to a state that permits first cousin marriage, marry there, and return to a state that bans it, your home state can treat the marriage as void.
Federal law adds another wrinkle. Under the Respect for Marriage Act, the federal government considers you married for purposes of any federal law, rule, or regulation only if your marriage “is valid in the State where the marriage was entered into.”1Office of the Law Revision Counsel. 1 USC 7 – Marriage So a marriage your home state refuses to recognize may still count federally if the state where the ceremony occurred treats it as valid. That creates a messy split where you are married for federal tax purposes and single under your home state’s family law.
Federal Benefits and Taxes
Social Security
The Social Security Administration treats a void marriage as though it never happened. If you were receiving spousal or survivor benefits and your marriage is later determined to be void, the SSA’s position is that the marriage never existed. No formal termination date is needed, and any prior benefits determination based on that marriage can be revisited.2Social Security Administration. POMS GN 00305.125 – Void Marriages On the other side, if you had been receiving benefits on a deceased spouse’s record and then entered a new marriage that turns out to be void, your earlier benefits can be reinstated, because the remarriage is treated as if it never interrupted them.3Social Security Administration. Social Security Handbook – Section 406
Tax Filing
The IRS ties your filing status to whether your marriage is legally valid. A couple who filed jointly based on a marriage later declared void may find that their joint return was based on an invalid election, since they were never legally married. IRS procedures allow the agency to adjust the accounts of both parties when a joint filing is traced back to a nonexistent marriage, separating out credits, payments, and liabilities that were originally combined.4Taxpayer Advocate Service (TAS). If You Didn’t Get Your EIP, Your Joint Return May Be the Reason Why If your marriage is annulled or declared void, you would generally need to amend prior returns to reflect your correct filing status for each affected year.
Confirming the Rule in Your State
Because the details vary so much, particularly around first cousins, adopted relatives, and affinity-based restrictions, the only reliable way to know whether a specific marriage is permitted is to check the marriage statutes of the state where you plan to apply for the license. County clerks typically verify eligibility before issuing a marriage license, and providing false information on the application is itself a separate offense in most jurisdictions. If the relationship is anywhere near a prohibited category, talk to a family law attorney in that state before applying. It is the practical step that avoids both a denied application and potential criminal exposure.