Like I said above, one of the issues is the lack of understanding the sister's motivation or trigger. It would make sense to not worry about the specifics of why if she only said it once. Instead, we're given a character that has constantly been telling MC to die and pretty much nothing else besides clicking her tongue at him. She feels like such an empty, nothing character for being the sole driving force for the plot of this story.

Secondly, his parents make no sense and should have at least gotten a throwaway line about liking the sister more or something. Instead we only get that his mom seems to respond when he talks to her and greet him in the mornings. But then, instantly there is only silence at breakfast. Not even the mom and sister are talking, so it doesn't seem like the mom is ignoring MC and only being nice to the sister.

MC asks his mom why his sister was acting like that and she just said it was a rebellious phase. This doesn't make sense. If a parent thinks one child is being mean to their sibling because of a rebellious, then maybe talk to that child? At a minimum, the mom should still be talking to both of them normally at breakfast if nothing caused his sister's attitude.

Finally, we get to the main character and this relates to the story feeling empty without enough details. Any criticism I have of the MC could be explained away by some details about their family life and his school life. We don't know any actual details about their life at home besides the times we see his sister tell him to die. If his parents were clearly favoring his sister, then I could understand cutting off the entire family forever. Then for his school life, we see him pass out from stress, a girl he talked to was sad she couldn't reach him, then he confessed. That's it.

And his su*cide note. What to say about this... It 100% just felt like a cry for attention and for his family to care about him. I say this because he had no thoughts about su*cide after he wrote the note. He packed his bags, got his money, then lived out of a love hotel for 5 days. Before he even went to live out of a love hotel, he thought about going to a friend's house but decided against it since he would be caught immediately.

Him writing a su*cide note was solely to be able to put that in the title. It would have been better off if he just said he was running away from home. Even after he got home and got attention and apologies from his family, he just ignored them. Again, if he actually seemed suicidal then that would be fine, but he really only wrote it in his note then never thought about it again for the rest of the story. Maybe if there were a couple of lines about maybe killing himself after running out of money when he was living in the love hotel, but it really just seemed like he wanted to run from home.

If someone tells you over and over again to go off yourself and you end up doing it, then they may learn the consequences of their actions.

If you ignore your sibling forever until you've completely lost your connection with them, you may realize their importance when you make your own family.

As for why both these actions occured, you can't recall. You only realize later on that it was s*upid and you should have not done it. I think that was what the author was trying to convey, how insignificant things may result into some overdramatic ending. The sister realised that her s*upidity could have ended the life of her brother. The brother learned that such actions in general weren't too good after getting his own kids and family.

But even if I try to summarize it, it doesn't really work out. If the author really tried to convey this kind of message, her pobably could have done it a bit better, give the brother his own little arc rather than time-skipping it and making him realize it in the end.