Law Education

In What States Is It Legal to Marry Your Sister? (None. Here's the Law)

No US state lets you marry your sister, and neither do Canada, the UK or Australia. Here's what the law says about sibling marriage, half-siblings, step-siblings and cousins.

LBLawyerBlogger Team· Published · Last updated 3 min read
In What States Is It Legal to Marry Your Sister? (None. Here's the Law)

Short answer: none. Every US state prohibits marriage between siblings, including half-siblings. A sibling marriage is void, which means the law treats it as if it never happened, even if a licence was somehow issued. Canada, the UK, Australia and nearly every other country have the same rule.

People often mix this question up with cousin marriage, which is legal in about half of US states. That's a different relationship with different rules, covered below.

United States

Marriage law is set by each state, but on siblings they all agree:

  • Full siblings: prohibited in all 50 states and DC.
  • Half-siblings: prohibited in all states, treated the same as full siblings.
  • Adoptive siblings: prohibited in many states, though a few allow it because there's no blood relationship. Rules vary, so check your state's marriage statute.
  • Step-siblings: generally allowed, since there's no blood or adoptive tie, though a few states have restrictions.

Separate from marriage law, most states also make sexual relations between close relatives a crime (incest). Penalties range from fines to many years in prison, depending on the state.

What about cousins?

First-cousin marriage is legal in roughly half of US states, banned in about half, and allowed only with conditions in some. Arizona, for example, allows first cousins to marry only if both are 65 or older, or if one of them can show they can't have children (Arizona Revised Statutes § 25-101). Some older articles misread this exception as applying to siblings. It does not.

Canada

The federal Marriage (Prohibited Degrees) Act bans marriage between people related "lineally" (parent, grandparent, child) and between brothers and sisters, including half-siblings and adoptive siblings. Cousins can legally marry in Canada.

Sexual intercourse between siblings is also a crime under section 155 of the Criminal Code (incest), with a maximum sentence of 14 years.

United Kingdom

In England and Wales, the Marriage Act 1949 lists the relationships within which marriage is prohibited, and siblings and half-siblings are on that list. Adoptive siblings can marry. The Sexual Offences Act 2003 also makes sex with an adult relative, including a sibling or half-sibling, a criminal offence (sections 64 and 65). First cousins can legally marry in the UK.

Australia

Under the federal Marriage Act 1961, section 23B, a marriage between siblings (whole or half blood, including adoptive relationships) is void. Each state also has incest offences in its criminal law. First cousins can marry.

Practically nowhere. A few countries, including France, Spain and Portugal, don't criminalize consensual sexual relationships between adult relatives, but they still don't let siblings marry.

The closest exception is Sweden, which allows half-siblings to marry only with special government permission. Full siblings cannot marry there either.

Why the law prohibits it

Courts and lawmakers give several reasons:

  • Genetic risk: children of close relatives have a higher chance of inherited disorders.
  • Protecting families: the law aims to prevent abuse and pressure within households.
  • Social and moral norms: the ban on sibling marriage is one of the most widely shared rules across cultures.

What if you find out after the wedding?

Because a sibling marriage is void, there's nothing to divorce. Either person (and sometimes others with an interest, like heirs) can ask a court to declare the marriage void. Children of the relationship still have full legal rights to support and inheritance from both parents.

Sources

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