I mean, the fact that Furuya is actually responding like a lolicon would explain why they're trying to correct him first: he's, by behavior, actually a lolicon... thought it doesn't explain why Souya wouldn't say SHE DIDN'T SEE HIM BEING A LOLICON BEFORE. Like, just establishing that it is a new thing, with a power that is undisputed like hers, would work to start trying othe avenues of approaching the issue. Guess him avoiding her for so long actually affected her thinking a bit, with how emotional she can get.
Maybe this power might be affecting the whole world hence nobody really picks up on something being wrong seeing that even the MC's devices got filled with photos out of nowhere and a bunch of kiddy magazines just magically appeared near his bed.
I mean, I don't remember if the sister had already checked his room, and the photos on his phone would've been private information that none of them would've know beforehand. For all they can guess (besides Souya), he might've been a lolicon for a while and just kept it well hidden.
And from the weird trial they're trying to do on Furuya, it might actually turn out that members of Exorcist HQ are trying to get rid of him by incriminating or implicating his involvement through this incident (now that sending the lolicon slayer failed to do so) or something. We've seen enough exorcists fear his power already, I wouldn't put it past the author to write something like that.