The right of siblings to marry is already being examined in other countries.
'I've married my sister - now we're having our second baby': Siblings who defied law plan to start new life abroad | Mail Online
Gay advocacy fully supports sibling marriage and it is one of the things they are working towards.
Full Marriage Equality: Frequently Asked Question: Can Siblings Marry?
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. LetÂ’s make it happen sooner rather than later.