Okay re-read through the series to remember what was going on and I think I have some theories:
1. Illya isn't the only one going through the loops, but is the only one aware of the loops. It's shown in the chapter that Soleil is being controlled by an external force that he is vaguely aware of the loops, but doesn't really have any agency. Other characters like Sylvia might be in a similar situation, since in the chapter that shows what happened in the brothel loop that she wishes she could do things over again. Ditto Soleil's friend Scion also makes a cryptic statement to Illya about not letting go of Soleil. Crow also asked her if she was the only one actually suffering, and in the brothel loop, one of the other girls getting trafficked also reminds her that she isn't the only one suffering.
That said, the loop might not be happening the same for everyone, or even happening in a different order for the various characters, like Al.
2. Just as Soleil doesn't have control of his actions, I think Illya doesn't have full control of her actions. There's many instances where her memory is stated to be hazy, and even instances between what is being said she did VS what we see her doing, but whether this is an effect of the world/loop or she's editing her own memory of events. For instance, the chapter where her mother kills herself, she dies telling Illya that she'll be okay without her, but the next chapter, her last words are she never once loved her, or last chapter where we see Illya cover her mother's mouth to prevent her from speaking, but her father states Illya had her hands around her mother's throat.
Crow has also implied that Illya has done something to deserve this and that she's hiding the truth from herself/the answer she gave him on her sin wasn't the real answer.
There's also how the staff treats her. her old maidserveant, Marge, is shocked that Illya even remembers her, Sylvia's maid that brought the key is straight up afraid of her, to the point of shaking, and the Steward even tasked her to keep an eye on her. In the first life, Illya implies that she did more than just freak out at that tea party that loop seems to revolve around, including intimidating other girls she thought might be going for Soleil, including the girl who becomes her friend in later loops. The second loop also had a lot of people implying she proactively went after enemies/potential threats to her Husband agressively and by any means neccessary. She's also convinced she's much worse than we've been shown.
There's also the fact that their parents never want the girls to be around one and other, even in this latest chapter, they both want her to not get too close to Illya, as if she'll hurt her.
3. Scion might be the force behind the loop/Soleil's actions. This is more gut feeling, between how he led the conversation they all had in the cafeteria and how he seems to be enjoying/egging on the drama between Soleil, Illya, and Sylvia. He seems like he knows more than he lets on, and when he's around, Soleil acts a bit different too. I think he might be from the same country that Sylvia's mother was the princess of, and maybe is trying to twist their story into one that suits him, since he mentions he's a foreign noble, and hides his exact title/name. He might even be related as a distant cousin to Sylvia.
4. Sylvia is not be as kind as she appears. We've seen before that Sylvia will hit on Soleil more outwardly when the two are alone, lied about Illya having another love in the Brothel loop, she has an affair and gets pregnant by Soleil in another loop, and may have manipulated the staff into thinking Illya was worse/crueler than she is, between Marge being surprised that Illya even remembering her name, and the maid who is afraid of her. This might just be the influence of the outside force implied to be behind the loops, Soleil's behavior, etc.
5. Crow is the outside force. Maybe he's feeding off her despair and when we see him being kind to her, it's to set her up for more despair later.
6. Crow is trying to stop the loops, but I don't know who he is or why. He kinda looks like one of Sylvia and Soleil's kids fromt he side story, but he also could being Illya's child from the second loop.