You know, this is actually a really interesting story about boundaries and expectations. The sister went ahead and made it creepy, but Ayano is just a young girl with no adult figures in her life. She's a kid who grew up too fast and who doesn't have anyone she can actually trust, and along comes a guy who feels like he's an older brother, or her father, or something like that.
But he's none of those things. She doesn't know how to feel. She loves him, but like... of course she does. He's the only adult figure who has ever tried to pay attention to her, make her comfortable, and get to actually know her. She's been desperate for this sort of thing. He's an adult who actually is there to re-assure her, give her comfort, and make her feel like she's actually got something like a family. Her love is affection for someone who cares, but she has no context to put it in other than what everyone around her talks about. So she sees him a bit like that, too. She's got a crush on her manager, just like how other girls have crushes on their tutors or famous actors.
Meanwhile, Fumi is dealing with some sort of heartbreak, but also genuinely struggling with a difficult thing. He's her manager, thus is trying to keep his professional obligations in line, but he also can tell that... well, she's lonely. She's 13, an only child. Her dad's out of the picture. Her mom's never home. She's constantly busy, constantly working, constantly on guard because she was forced into the spotlight as a child. She has to be careful with who she's seen with. Has to be on guard for all sorts of things, apparently including pedophiles. He sees all that and realizes that she doesn't need another person who's just connected to her because of her occupation.
But he has to be careful, because she's a child. He's an adult. He's her manager. There's a clear power imbalance. He has noticed her clear affection for him but doesn't know how to react in a way that's productive and socially acceptable, and to top it all off, it seems like he's got a genuine trauma around something that recently happened, cause he'll like... freeze and have a panic attack sometimes.
I donno where the story's going. I certainly don't like the sister's idea, even though that's what I've learned to expect out of manga. But there's real meat here in a relationship between a lonely child and an older male figure trying to live up to her confused and uncertain expectations.