Who Wasn't a First-Time Human, Anyway?

Synopsis: When people are too capable, they get promoted. When a System is too capable, it becomes a Host. On its first day as a person, the System encountered its lifelong nemesis! They have enough grudges between them to talk for three days and three nights. It's both our first time being people, so why should I give in to you! Damn it! That's your little sister! Note: 1. The blurb is for reference only. The actual story is what counts. 2. A companion story to the sibling novel next door, I Became a Relative of the Cannon Fodder Villain. This is the sister's first-round story. 3. In a way, this is probably a story about a brother and sister tearing into each other. 4. This novel is also called My Dumbass Brother and The System's 365 Days as a Person. I'll add more if I think of anything. Theme: Harmonious sibling relationships, building a beautiful family together