Responses
There's no logic in manga.
She didn't know about the inspection, so I suspect she didn't think *anyone* other than her twin would find out about it. It's still a risk, but it's not like he'd be showing it off after he saw it. It's one of those things that's fine 95% of the time, this was just a bad day.
The sister was actually pretty stupid for putting the vibrator in her bag. Did she not realize that people would think it was hers and her brother's?