That's exactly what made the whole anime lame at the end: Moments like this being half-assed. It would have been so much more fun if he had truly crushed his sister. And then she would have gone full-villain mode, meaning more fun for us.
It's like the writer of this story intentionally kept himself back, trying to make it milder a story. For the worse, imo.
Oh, one thing I really have to rant about, too: Alya is SO helpless without Masachika. He tells her afterwards "you made this happen, Alya", but that's bs. She would have completely faltered without his help. Kinda ruined Alya as a character and turned her into a pretty bimbo. Which is weird, because when the anime started, she was portrayed as an all-capable character.