@Ceereus
That is because she is weak.
In a story, a character will act with a certain personality and will not change until an External Force alters them(Usually the new Protag in a reincarnate story)
The Heroine in "The Villainess loves" was a nice, dumb, cry baby; she was not a bad person but that is her whole personality from beginning to end in her story.
That is her personality and even if you think she was a disgusting fake for not "caring" or doing anything for her "friends", that was how that terrible writer chose to make her.
If using that as an example, Irene is a weak willed but kind girl that is too soft to make anyone upset.
Even when Sylvia apologized, the first thing she did was leave and Cry(Likely out of happiness) which shows how childish and easily emotional Irene is.
She probably will not change until her older sister(New sister) educates her.
Basically a character will act the same unless something that diverts the "story" or "character" changes them.
Edit-> Irene was likely kind to her sister and never did anything out of place but clearly was not aware(Probably this one) or did not know how to deal with her sisters jealousy.
Once reincarnated in a book or game, it is basically a real world but that will not change how someone was written to act from the beginning and then the World will fill it in from there.