I guess that's one way to get your older sister to play with you.

Ai-chan: "So, you have chosen death."

So Imouto forces the return of the Oni again by not thinking . . . .

Y tho...

O-(cup)-ni out!
Appropriately, the page she's reading has ア at the edge, which is the sound I expect she would make.

I think it has something to do with the kimetsu no yaiba manga that Ai-chan's reading (would be related to imouto's Nezuko gag and the Urokodaki/tengu mask, along with beating 'Tanjiro'). Is there a line where Tanjiro thinks someone is getting turned into an oni? Imouto is probably thinking the exact line but referring to Ai-chan.

Also, per the author's line, it would be the 3rd time the sisters get a little carried away doing a manga/anime reference.

Note: The scene of Sakonji questioning and then slapping Tanjiro becomes an exploitable meme in Japan

https://dic.pixiv.net/a/判断が遅い

It seems that people forgot Ai-chan turning into a Oni here also - post #3405739

We don't see the sister enough. She is amazing.