The little sister I'm talking about is MC half-sister in the past life.
She hated her brother for the reason that their father divorces their mother but MC decided to follow the father instead of the mother who cry when she was divorced. However, the reason MC follow their father because the father knew that the sister is not his child, and the mother had been cheating on him for years, yet the father never makes an issue of it and want to end things peacefully by having a divorce and let the mother marry the man she having affair with. Which not even a year after the divorce the mother marry the man she having affair with all this time. That said the mother does feel guilt, which is why she always crying asking for forgiveness in her heart from her former husband and son., doesn't change the fact she is a bitch for not telling her daughter the truth though.
The little sister never knew about this, and simply thinking that the father is just an asshole, and hated her brother who followed such a man, even though MC followed his father because he doesn't want to leave his father all alone. On top of it, this shitty sister of him tries to make MC love interest the main girl to broke up with him thinking that her brother doesn't deserve such a good childhood love interest. She also tries to get MC's love interest to go out with the half elder brother from her 'real father' previous marriage, thinking that this older brother is a good person. Unknown to her the apple doesn't fall far from the tree this brother of her is an asshole who plays around with a girl and arrogant as hell deep down, he always like the MC love interest because of her look so using his sister he actually tries to get with her. Well MC love interest never likes him so she always keeps her distance regardless of how well mannered he acted in front of her.
But what makes me hate her is because she actually caused trouble in the future, and make me drop this series. Just read it, you can only get hate from reading her line of thought, as well as her other half brother.