What’s the point of loving her sister in this life after she went back to past again she...

Oh now I see it

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Your questions have already been explained in the story:
- Other characters have noted that while Cezanne is kind, she doesn't actually pay much attention to anyone else except her family. Hence why people had a difficult time with her in the past: because her kindness is just a touch too shallow. She probably just thought that Fleta had lived in a much better life, not knowing that in her ignorance and obliviousness, she actually suffered in silence.
- Her hypocrisy has also been called out — by her sister, no less. And Cezanne has acknowledged it too. It may have started out as guilt of what happened to her sister in her past life, along with fear of her powers (as evidenced in the early chapters), but she did grow genuine care towards Fleta. And mixed with those emotions, she doesn't want her to go through pain and hardships, for Fleta and for herself.
- In line with the previous point, Fleta doesn't want to go anywhere without Cezanne, having grown a strong attachment towards her. Hence why both her and Cezanne refused to just let Fleta be taken alone with Kilia in his kingdom. And after realizing that her parents may never change their mind regarding Fleta, along with Vivianne's antagonism, she decided that it's best for her to move away from that toxic place, again for both herself and Fleta's sake.
- About Kilia: it is possible that he had never known the truth about Fleta in the past life. Remember that the only reason that he knows the truth about Fleta.. is because Cezanne told him about it. When he asked during his first meal with the ducal family, nobody was willing to talk about it, even going as far as avoiding the topic. And Cezanne who did not know of the truth at the time, might've done the same. And eventually Kilia returned to his kingdom without finding out about Fleta.