Pixiv title:

ふーはぁー…すぅぅぅっぷぁぁぁっ。

Commentary:

忍び寄る妹、そして飛び散るしずく。危うしおにぃちゃん！……次回『決定的瞬間』は水曜夕方五時投稿です！……と前宣して追い込んでみるけど約束守れる自信はないので、水曜日にキャプションが消えてたらそういうことだと思って下さい(〃▽〃)

I suspect the trap just got trapped.

The title is essentially onomatopoeia of the boy sniffing his sister's panties.

You know... I'd expect the sister to start getting REALLY creeped-out about this around now.

He's basically continually stealing her underwear, wearing it, and masturbating to it at this point, even after being caught. That's stalker territory right there.

Seriously, girl, get a deadbolt for your bedroom door and sleep with a taser.