Honestly the constant incest baiting is starting to get annoying, also he happily bathes with his aunt but refuses to with his mother? Like I would get it if he was ok with both or not ok with either but those are some odd standards.
I took it to mean she hurt him with the "charm" which she absolutely did hurt him with it. What you're talking about is oddly specific, ISTG if this turns out to yet another untagged spoiler on this series I'm going to track you down and flay you.
from all the stupid sh!t the series has thrown, him actually bathing with the aunt is the least concerning or surprising, what wasn't neccesary was him flashing her, him not bathing with his mom is because of his trauma, his severe fear of her rejecting him is one of his strongest trauma, and the bath, which is getting naked or well, his weakest form is clearlyto show it (manga prefers to use the incest bait there though) ... while the auntie is the only one in the whole story that didn't gave him the feel of betray ever, and as we know his emotion control/horny teenager tendencies is stunted, so he doesn't realize it's not normal at all to keep bathing with her, but she still gives him the safe feel as he was still a children. BTW he shouldn't smex her, my rationale only is for the bathing scene not go further (like the reverse bunny suit)
tl;dr = she's the only one that hasn't betray him so he still goes child mode with her.