A really nice and funny story. Despite the title it's not really ero. The boy isn't trying anything with the girls and he's very thoughtful. Despite being senin, the art is very bubbly.
The second arc ends too nasty. It's like in Otaku no musume, when that guy marries the girl he raised(a person with responsibilities to another person's welfare) but twice as bad, like a story about a King trying to marry his daughter except those stories don't try to hide it and have him acting like a good dad, and he's not usually let off like it was an innocent thing. That's the kind of thing that needs its own story, just throwing it out there is messed up. I was actually seeing some similarities with Bakemonogatari, all the few people helped are girls but I was thinking it wasn't as fetishistic but that arc...
Plus it was a more interesting, dramatic and real story(like the first arc) before the twist that seems to be popping up a lot in shounen and seinen works(seriously think a twist ending is the wrong way to utilize or insert these types of characters but that's not whats wrong about this anyway so that's moot). Stuff like this becomes a spectre or gross lingering after taste. Maybe It doesn't ruin it but its something to put up with while enjoying it for awhile till you forget. I guess in the long run it might be minor. I read this at night(five days ago) and it bothered me even after I woke up the next day.
She hurt one of the characters twice and was willing to leave her in half of one of those "and I must scream" states and she didn't even really get reprimanded(This comic was way too soft on her). She got reprimanded for much less batshit insane crap(like leaving home for college). Maybe it was suppose to be a joke and funny but it's too messed up to take lightly. And it's put in so randomly since everything else she did falls under normal behaviour.
I mentioned this in passing but as much as you can stretch it to be about the overall theme of masking emotions, this really is unlike the entireity of the manga upto that point. It's like I'd like to be able to enjoy reading it without it trying to damage my psyche. And all of this is a couple of pages in a single chapter...
I'm not saying don't read it. Think of it as a warning that might not even affect you. It's too bad I have no way of telling these mangaka that a good, well told story doesn't need a cheap shock, just look at the first arc. The second arc took a serious issue and threw it out, and then resolved it be ignoring it with no consequences, shoddy but that wouldn't matter if it wasn't so distastful and unnecessary(maybe if they had established it at the onset, it'd have still been messed up since she's the elder one but it could have actually been used for laughs then). Like a crappy version of one of the episodes of Paranoia Agent with the family and the hidden cameras.
Basically this was really creepy, and the girl was a psycho(with a yuno-like vibe, worst with who she aims it at), and the manga just doesn't truely deal with it. It's just brushed aside.
This was a good manga upto that point, and it'll probably be a good manga after that point but to paraphrase from nekomonogatari; This bad part will always be there, even if things get better later, it won't change that.