It isn’t only the fact that her sister was (aspiring to be) a knight, but the sister’s fans and followers were knights who blatantly disregarded Ana, made up rumors about her, slandered her name, and used their position for all the wrong reasons. Also her powers were hinted at in chapter two and even used in chapter four or five on some dumb fanboying knights blocking her path. (Insert obligatory fuck the sister speech here.)