Because some people havent play the expansion, so there is no way they know anything outside what they have seen in cinematics. You have to actually play the campains to learn who the Accuser is, or you see Winter Queen call Ysera her sister pet. There is too many side stories to know them all if you dont actively play.
Btw this could be figure of expression, just like how the Archon calls Denathrius brother, so its not 100% confirmed Elune have a sister.