tbh, at first I really can't grasp the immorality between them, because they were practically almost strangers. I can't even feel the siblings vibe at all.

If you can turn to a blind eye about the ML being FL's younger half brother, they look like strangers living in same house, or even stepsiblings, if this even make sense. I would say the feeling of immorality isn't very high, until ML started to call her sister in bed.

how FL don't shy away in seducing him to sleep with her also make me like the FL. she is shameless, bold, cold-hearted and even kind of cruel. she know he had feelings and thus seduced him.

although personally, I like the ML more, he is devoted and kind, helplessly in love with FL but I somehow feel he is going to the yandere territory with how the FL keep breaking his heart....

well, after reading until the end, I honestly feel kind of bad because ML was actually a victim of the situation. he did no wrong. FL used him for revenge when ML knew nothing, and she even heartlessly left him at some point (they reunited later though).

ML feelings towards FL is written clearly but FL's are subtle. until the ending, it seems that she ended up falling for him but didn't want to admit it.

it's a happy ending, and the family issue was wrapped up pretty well (although it was awfully convinient since the dad died). I pitied the mom a bit, but as she said herself, it might be her own 'retribution' of her past mistakes towards the FL's family.

though honestly, she isn't very bad compared to the stepmoms from the other chinese novels, and the author really gave her a well written reaction about the situation when she found out about FL & ML's relationship.