Calling it a script is overselling it. It's basically a single IF or switch statement. And your solution already means the game would be testing if Sebille is present anyway. You just need to check that she is present as a companion, and then you can force the response without player input.
You not only need to check whether Sebille is in the group, you also need a specific time during the dialogue in which she delivers her line(s). And not only her, but everybody. So you also need a kind of conversation order for each dialogue that determines
a) which character has automatic lines
b) which character talks at which specific time/state in the whole dialogue
Let's say you have a four character party and you start a dialogue. You need a specific script/check in order to determine who is allowed to talk and in which order.
There are several possibilities of how such a system could be implemented:
In the solution with the least additional effort involved everbody would deliver their line in a predefined (or arbitrary) order at a specific time that is the same for all dialogues (e.g. after your main character chose his own race/trait dialogue lines). But this would make the game very static (everybody shoots their lines right one after another) and the dialogues would become very, very long (every dialogue would involve all the lines of all characters then no matter what).
Every other solution would require more thought and more work, especially if you want to treat dialogues individually, e.g. by only letting singular characters being involved in certain dialogues. But then you have to go through each dialogue individually and you have to set individual flags for whom to talk and when to talk. And more than that, it would require the devs to carefully think about which character to be involved in a certain dialogue and which not. Why should Sebille comment a certain line/dialogue, but not Ifan or the Red Prince? And why only Lohse in that other solution? Questions like that needed to be answered for every possible dialogue. And what about the characters that are not involved? Will their existing dialogues just become invisible/ditched (at least for this playthrough)? Will they stay just silent without any chance to address a topic?
And beyond these in-process questions a system like that would be against Larian's stated vision that you can explore all of your characters dialogue options, even in SP.
I know I've brought this up before, but that's not how dialogue editors work. I promise. Any line can be conditionalized. It's literally just a matter of adding a branch that only runs if the relevant character is in the party. That's not a "script," it's just a very simple condition, requiring no more work than any other line of dialogue.
But it does. It's not just an option that pops up, it's an automated process. And as such it needs a specific start condition. It's not just another dialogue option to choose from. And like I already explained above, we're talking about up to three companions here, not only ones. So such an AI-controlled system would require the devs to carefully craft dialogues in specific branches and processes. And it would require them to redesign every dialogue which specific participation states and starting conditions for automated dialogues of your companions. That's not what I call "a simple condition", it's at best a set of conditions - a set of conditions that could be completely different for every single dialogue in the game.
