It seems like you're discussing a situation in a visual novel where the main character (MC) unexpectedly介入 (barges in) a scene, interrupting a family dinner, and involving themselves in a romantic subplot with the little sister. You're questioning the logic of the MC's actions, considering it's supposed to be a linear, predetermined story.
The confusion arises because, if the plot is fixed and the little sister's romantic ending is positive, it's unexpected for the MC to disrupt the family dynamic, especially if they're aware of the story's progression. However, you also mention that the MC's behavior could indicate they have personal reasons beyond just following the plot, such as their love for yuri.
In your comparison to other stories where the protagonist is aware of the plot, the MC's lack of surprise suggests that the game might not be a straightforward adaptation, which adds to the mystery of their actions.
The character interactions and predictability of the story seem to be a point of dissatisfaction for you, describing them as stereotypical and boring. You find the MC annoying due to their complaints despite knowing their elf master's strength.
In summary, the MC's intervention in the dinner scene appears to defy expectations given the established plot, and their actions raise questions about their motivations and the nature of the story. The story's formulaic elements and lack of surprises contribute to your frustration with the narrative.