I feel like everything points to her being the younger and more timid sister that didn't curse the hell out of everyone. What little I've read for this series on the Wikipedia article for it (general Wiki, not the fan wiki for the series) lists Daleth as having this infatuation with Walter that we've been seeing and Sade being dead, as far as can be told, though traces of her magic still existing, such as the curses she's placed on people (the end of her little description blurb saying that's 'a sign she was not completely destroyed yet').
Everything about this arc is shit, starting with it coming completely from the left field, and I honestly hate it. This arc alone lost the manga two points out of my rating.