Additional Reading, not worth retyping fully here:
What if neither the protagonist nor antagonist wins?
I feel my protagonist is too "detached" from the main plot. What should I do?
Protagonist - Character who is working towards one or many goals. They are pro-active. They do things.
Main characters are not necessarily protagonists. Protagonists do things. Main characters are around a lot and tend to be equated as the point-of-view-character.
The primary example from the above questions that I think applies here is The Great Gatsby. Nick is not a protagonist. He wants to achieve almost nothing. He is invited along for the ride. But its a really good ride. Others have referred to this as well in other stories. So your main character need not be the protaging sort of character.
This is the rule: Stories should be compelling, they should have an arc, and there should be protagonists and main characters almost all of the time. Almost. But, what people are really after is an interesting story. And the point you choose to tell it from should be the most interesting vantage you can; or at least the one that is interesting and makes the point you want to make.
If you can best tell the kind of story you want from your 3rd not-fixer position, then do so; but you can probably learn things from other stories that have done this successfully. The first is that your story shouldn't be boring, and that if your character is largely boring, then they should at worst take on the role of a narrator who is showing the reader interesting things merely by being in the right place. I'm not saying that your character should be a robot. She should have stakes and feelings. But if its better to have a character that stays on the sidelines so that you have some distance between your reader and the characters being observed, then by all means do it.
Like with all things in writing, if you want to stray from what people often do, you'll find you have to work exceedingly hard to get it right. You're going to be telling Aster's story anyways, so Sirena had better bring something to the novel, some perspective that makes it worth having followed her the whole time. That could be many things. She could have a vantage on a number of proceedings. She could be personally affected by traumas. She could have more mundane, relatable concerns and yet have to deal with this. She could be a red herring. Maybe everything indicates she'll be the Harry Potter, but we're just reading about Hermione afterall (there is nothing wrong with Hermione). Some advantage you might also get is a limited perspective; being able to hide some things allows surprises you might have spoiled by choosing a more active protagonist; inside the circle everyone knows the plan, but to an outsider its an unexpected turn of events.
I can see lots of ways to play this, but you're also going to have to drop hints along the way so that your readership doesn't hate you when they get to the end. Sudden, but inevitable is what you're going for here. It can be a surprise when its actually Aster, but when the reader goes back over the material, it should make a lot of sense. And it shouldn't be disappointing. Figure out how this is a victory for everyone, even Sirena; or at least how its interesting for everyone.