ehhh, i respectfully disagree, although it could be a possibility that's a little but of a stretch, do not underestimate a mother's love.
Hm, but if we take away Sangwoo's vision in that previous chapter, or attribute it only as some crazy dude's insane romantic edipus complex, nothing in chapter 40 speaks of abuse other than the mom's reaction to the love letter, and even then she quickly snaps out of it and realizes the wrongness of her attitude, in a way that can make us say she manages to keep things within the realm of "normal given the circumstances". Up to the part were she starts talking about secrets, everything else could be attributed to a child feeling his privacy and personal space intruded upon by his mother, in other words to Sangwoo having a somewhat overbearing, nosy and hovering mother, but that's still a long shot from her being abusive...
Be as it may, what is clear is that it's enough to make us suspect, sure, but up to this point it's difficult to say whether the author is intending for this suspicion to become. Either she's playing with our minds a little, giving us enough signs to keep us on our toes but still nowhere near to let us confirm anything, or she's completely unaware of the fact that any of her readers could have taken the scene about Sangwoo's edipus fantasy in that previous chapter as anything more serious than a lunatic's exaggerated version of an edipus fantasy, and that they could've misinterpreted in the way many of us appear to be doing.
Ahhhh I sometimes forget that the author does it as their POV where as other mangas it’s more a fact... I get what you mean though... although the mothers definitely not righ in the head and I think if she didn’t actually abuse him she gave him the creeps yknow? As an adult when I get unwanted attention I feel awkward but never mind if it was a parent? And you’re a child. I dunno but I do think the authors messing with us a little bit lol. Always so scared to read the next chapter
I really do think she sexually abused her son, or at least gav him creepy vibes yknow? And maybe the reason why sangwoo is so hateful towards bumis attitude over his uncle is because he can relate? But instead of being the victim he killed his abuser? Despite the love he had for her? A child locking their door or not wanting their parent to help them change can be alarm bells, usually kids just get annoyed but he seemed fearful. He resembles his father a lot as well and so I think in her deranged way the mother replaced her husband with sangwoo.