They changed the plot a bit.
In the original MC defeated the classmate and the sister, (gal).
When he got invited by the mamily they were annoying like here, but MC didn't just sit there and take it like a wimp, this author is making a poor adaptation with a dumbwit MC, in the novel he shows having passed a decade in a medieval hell world.
At the meeting the family brought up making the classmate the sacrifice after her defeat, I don't remember the details clearly but I think it was MC that brought it up when they said that Mio was of unknown origins and the sacrifice should be someone of higher standard, he was just trolling but it worked, it showed her being super scared from becoming it, when in the manga it looks like she was already the decided sacrifice, that is very important so I can only see this chapters being stupidly short as the reason for it.