Unpopular opinion I'm sure, but considering the state of demonkind in this world, I don't blame her for trying to kill Ellen to save a whole bunch of trouble later. Probably be turbo pissed if she finds out who her brother was.
From what I've learned this story will likely tank at around ch 187 of this manwa(likely around 326 web novel).
The reason is because a lot of people seem to hate the novel around this point as it becomes less character driven.
Ex:
Ellen and Charlotte ends up telling Bertus about Mc’s identity rather than confronting him first(yes it understandable that they would feel betrayed) but rather then being competent in interrogating themselves they rely on other people just for the 'plot'