After MC absorbs the Queen of Poison within him and turns into his adult form, he decides to leave and go on a journey. He runs into his father as he is leaving. After some insults from him, MC being very prideful and confident now (
his personality change can be compared to Jinwoo from Solo Leveling) taunts him and they begin to fight. It's not one sided as MC is using the family styles in real battle for the first time, but it's clear that MC is by far stronger. His father being desperate uses his ultimate skill but MC counters it and knocks him out with the most basic one. Even though MC told him he would kill him, he lets him live out of respect to his late mother.
As he is leaving he runs into his sister and a few guards who noticed the battle. She starts insulting him, not believing he defeated their father, he just calmly knocks out the guards and walks by her. As she gets ready to attack him he releases just 10% of his magical power, which creates a shockwave that almost beheads her. She is scared as hell, and he just tells her "be glad you kept your head, next time there won't be any mercy", something like that, and then he leaves for good.
And that's it for them... All this will happen in the first 10ch of the manga i'd say. I also checked a bunch of the future chapters at random, even the ending, just to see if they have more appearances, but no luck. It's like the author forgot about them
The WN is complete, well the main story is... There are still some epilogue stories coming out, maybe the author remembers them one day lol
EDIT:
I somehow missed an interlude chapter which had a bit more about them. These chapters are not numbered and used just to describe stuff, so i ignored them not noticing that this one was actually a continuation of this fight above.
Anyway, the father wakes up in his bed and is informed he was out a whole week. His butler explains what happened, and he is shocked his son is so strong, far beyond his sister. He sounds a bit remorseful, but still labels him an evil being because of the Queen of Poison within him. He feels he is somehow weaker now, and finds out his son poisoned him. This poison sealed most of his power, and he can't do nothing about it.
The butler informs him then that his daughter meet her brother and since then hasn't left her room. They go to her, but she actually isn't there, only a note saying she left to kill her evil brother.
I have no idea what happened to her. There are no chapters about her after this, or their father. I read like 70% of it, searched a bit of the ones i didn't get to yet, even double checked some of them, but no luck. Google translate is not the most reliable, but it's still good enough. Also her name is not so easy to miss... I could have missed it if she was just mentioned slightly, but i kinda doubt it.