The following is based on my understanding. I’m not at attorney and this should not be considered legal advice.
Can siblings marry?
I’m not aware of any government that will currently marry full-blood siblings or recognize a marriage of full-blood siblings; rather, if it was discovered by the authorities after an official marriage was formed that the spouses were, in fact, siblings, the marriage would be dissolved and considered invalid. If the spouses knew they were siblings when they married, they would be subject to prosecution. If they discovered the genetic relationship after getting married, they would have to file for an annulment or dissolution or risk prosecution.
Where sibling consanguinamory isn’t still banned by law, siblings can have a wedding ceremony and live the married life, although under discrimination, as their government will not recognize their marriage and they will not get treated equally.
Sweden will legally marry half siblings under certain circumstances. I’m not aware of any country that currently has more progressive laws or laws as progressive as Sweden.
Some siblings report that they have been able to get a marriage license in places like the US based on the ignorance of the authorities, such as the siblings being born in different states or countries and/or not having a shared parent listed on their birth certificates. However, if the laws of that location do not recognize sibling marriages as valid, or if consanguinamory is illegal in that jurisdiction, a marriage license is a potential piece of evidence that can be used in criminal prosecution, and that’s sad.
[UPDATE October 2014: An anti-equality lawyer who has practiced in at least two US states insists that there are at least a few states where same-gender siblings should, by the existing laws, be issued marriage licenses.]
If siblings want to get married, they should be free to marry. Inequality, based on prejudice, is counterproductive. All over the world, there are siblings living as spouses; there always has been, some with the knowledge and support of friends and family, some hiding the full nature of their relationship. Sooner or later, full marriage equality will be in place in more progressive places, allowing siblings to marry without discrimination or fear of prosecution. Let’s make it happen sooner rather than later.
This question may be asked many different ways. Can siblings get married? Can siblings marry? Can a brother and sister get married? Can a brother and brother get married? Can a sister and sister get married? Can a sister marry her brother? Can a brother marry his sister? Can a sister marry her sister? Can a brother marry his brother? Can you marry your sibling? Can you marry your brother? Can you marry your sister? Can two sisters marry? Can two brothers marry?