Every state forbids you from marrying any of your ancestors or descendants, including your brother, sister, half-brother, half-sister, aunt, uncle, niece, nephew, mother, father, grandmother, grandfather, great-grandmother, great-great-grandfather, great-great-grandfather, great-great-grandfather, your great-great-grandfather, your great-great-grandfather, your great-great-grandfather, your great-great-grandfather, your great-great-grandfather, and your great-great-grandmother.
Even if your mother is alive, but is not able to have children herself, it would still be illegal for you to marry her. Her genes would be passed on to you even though she could not physically produce a child with you. Because genetics are important, some states also forbid you from marrying someone who has a gene mutation that causes an inherited disease. For example, Virginia prohibits marriage between individuals who are genetically related because of differences caused by mutations. Similarly, Alaska, Arizona, California, Colorado, Connecticut, Delaware, Florida, Hawaii, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Utah, Vermont, Virginia, Washington, Wisconsin, and Wyoming prohibit marriage between individuals who are genetically related due to differences caused by mutations.
In some states, there are exceptions to this rule if your mother has given birth to other children, called "stepchildren".
Because some individuals seek to control the lives of others, it is still unlawful to marry your half or full sibling in all fifty states in the United States. There is no reason to deny any consenting adult the right to marry, and our laws will soon catch up. In the meantime, because of the potential for harm to both parties, we recommend that you not try this method.
The legal definition of marriage as only being between a man and a woman was established by the Supreme Court in their decision in Obergefell v. Hodges (2015). This ruling found that denying same-sex couples the right to marry violated their constitutional rights. However, before this decision, certain states had already granted marriage equality, so although your sister may have been able to get married in these states she would not be able to find a place on her wedding license that said "husband" or "wife".
In most states, there are also restrictions regarding the relative age of spouses. For example, in Alabama it is illegal for someone under 18 to marry. It is also an offense for someone under 16 to engage in sexual activity. So if your sister attempted to marry someone under 18, they could be subject to criminal charges.
In addition, some states require one spouse to be alive for the other to be married.
Second cousins are legally permitted to marry in every state in the United States. However, marriage between first cousins is only permitted in around half of the states in the United States. Overall, marrying your cousin or half-sibling will be influenced by local regulations as well as your personal and/or cultural values. For example, some people believe that they can inherit mental illnesses from their cousins; others may feel that kinship is too close to justify marrying your second cousin.
In most states, a person has to be at least 17 years old to get married. Some states require that you be an unmarried person for certain periods before you can be married. These periods vary but usually last from one year to forever. When you turn 18, then you can get married without any problems from legal perspective.
It is important to remember that once you are married, your spouse becomes part of your family. If something were to happen to your cousin or half-sibling, your spouse would need to be notified so they can express their feelings.
Marriage between first cousins is very common in countries where many generations live under the same roof (such as India). It is also popular in Africa, the Middle East, and Central and South America.
In Europe, there are laws against marrying your first cousin. The prohibition exists in all European Union member states.