AnswerIt appears that throughout the U.S. and in most of the world, parties that are in a prohibited relationship such as direct descendants or siblings, including adopted (by law) relationships are not allowed to marry.
Apparently, most locales believe that "as long as two individuals have two common parents, they are considered full siblings, regardless if they have been adopted."
The wording of the Colorado statute regarding prohibitive relationships does not include the word "adopted", so adopted siblings who want to marry one another may want to check further with a county clerk there.
Some reports state that Sweden allows marriage of two adopted siblings, but we have not been able to verify that information.
From StarsofDavid.com: "Theoretically there is no prohibition to marriage between adopted siblings. In fact, one such case is described in the Talmud. However, most authorities would forbid such a marriage because of unseemliness."
The assumed reason for these laws prohibiting marriage between adopted siblings is to prevent creating perceived "family chaos."
Proceed to the next question.