Well princess, you are one of the reason Celia was no longer smiling. Her favorite student had to run away because you and your gang kept discriminating him.
@spamtrashed I haven't really read the LN. But this is pretty much what happened as explained up to ch 35.
Haruto's parents got divorced when he was a kid. He went with his dad to move to a different city, leaving behind Miharu and his sister (Aki).
When he was of high school age, he went back to the same city. Miharu was supposed to go to the same high school. But she disappeared on the second day of high school. Haruto went on to college, died, and got isekaied.
In the present moment, we found out that Miharu was isekaied back then on the second day of high school. Basically there's a different time flow between Haruto and Ayase. For Haruto, it was 3 years of high school + 1~4 years of college + ~17 years of isekai life. But for Miharu, it was just a couple seconds between disappearance to isekaied.
We also learned that all this time, Ayase was still close to his sister Aki. And that Aki's mother actually remarried, and now Aki has an older step-brother and a younger half-brother. When Ayase disappeared, she was with four other people: Aki, the older step-brother, the younger half-brother, and a female classmate.
In ch 2 manga, Haruto never mentioned that his sister disappeared. Why? Idk. I'm guessing because he was more excited about reunion with Ayase. Probably he and his dad didn't really keep contact with his sister and mother after the divorce.
Also I just peeked the LN, and they only mentioned that there are other students who disappeared.
"MC's sister's older brother got isekai'ed in front of her"
And as I mentioned earlier, this refer to her older step-brother.