ponyclopstash asked: I recall one of the MLP writers on Twitter saying something about a book coming out that would explain the relationship between the royal sisters. Considering that it will be an origin story of some sort, that perhaps could offer some insight into the relationship further when it comes out. Course as far as the show and comics are concerned, I think there is enough proof already of them being sisters the way things are right now
I don’t regard these “supplementary materials” (comics, etc.) as strictly canonical (probably because I don’t take the time to read them XD). But this is an interesting point.