Board Thread:Speculation House/@comment-11188061-20131114110913/@comment-16502426-20131209085807

Issues:


 * Monty has confirmed that they are in fact blood-related


 * Monty never actually said that they aren't full sisters (that was a misquote)


 * Monty has confirmed that they are not cousins


 * Monty has confirmed that they are not step-sisters.

So this pretty much leaves full sisters and half-sisters. I'm inclined to believe that they're full sisters due to his "They're sisters. Like sisters are sisters" tweet, as well as the "When did I ever say they aren't sisters?" tweet.