If, by some tiny little chance, Aika encountered one of her half siblings, what would be her reaction and their reaction too?
It would definitely be a tiny chance like you said. Mostly because she's never actually seen her father, so if she met someone similar to him, she likely wouldn't be able to recognize them as her half sibling. The only tell would be the eyes, but it's still fairly rare to inherit higher being eyes 1:1 like she did. You can see it in the Grimm x Vyrm children, Lewk is the closest to having eyes like Grimm's, but they're still not exactly the same; meanwhile Aika's eyes are identical to her father's.
That aside, if she did meet someone who's her half sibling, maybe they would reveal they're the child of Mystral so that's how she would know, I think her immediate response would be mistrustful. She already struggles with a bias against gods and demigods because of her hatred for her father (a bit of hypocrisy considering she was a victim of such prejudice), and standing in front of someone directly related to him would make her ask questions. How close are they with him? If they know him at all, what if they're even a bit like him? What if his cruelty runs in their blood and they're a danger? Of course, those questions would only expose her own prejudice, but it would also potentially reveal fear and insecurity about herself. That deep down, she's as cruel as he is, and it's only a matter of time before she hurts someone.
But there would be a part of her that would be happy. Just a small part. That she's not the only one who has to suffer knowing their father is such a monster. That there is a chance she has a kindred spirit she could talk to about things no one else would understand. Maybe, if she got over her initial doubts, she would find it comforting to have met them. Maybe she would even find it in herself to try to build some kind of bond. At least a part of her would want that. The question is, if the hatred of her father would be stronger than the love she could have for her half siblings. A question she would struggle to answer.
How would the half sibling react? That's much harder to answer and greatly depends on what their backstory is. If they suffered a similar fate, someone who was abandoned by him, and maybe even by their mother, I think they could feel a similar joy of finally meeting someone who experienced the same thing at the hands of the same higher being. Or maybe they would hate her, for reasons similar to hers, or something else entirely. Maybe they would want nothing to do with her. It could really go in any direction.