^TBF from how they talked about it sounds like the whole bunch of dragon(?) siblings don't get along particularly well. Dusk, too, can be (by her standards at least) quite amiable to mortals she takes a liking to but her reaction to her big sister was... yeah. (Also lmao that one voiceline; "-- Do you have time to help me burn Nian's mountain of garbage? She is denigrating the very concept of cinema. Please.")
We'll no doubt learn in later events whether that's #JustDuskThings or if they're all like that. :P