Conclusion

You can’t choose who you love :(

but imagine how the older sister felt when she read her younger sister’s blog. the only person she has love thus far in her life (as it was before she fell for tougo) literally said she would take her to hell if she had to.

Also, it is not like she stole her sister's boyfriend while she was still alive. She passed away, and they deserve to be able to move on with their lives and be happy. It is as Tougo says, "dead people don't hate nor feel sadness". Haru is no longer there.

They did not get together while the younger sister were still alive (nor doing anything shady while she’s still alive). The older sis DID NOT STEAL her sis’s boyfriend. The man is a human, he cannot be ‘stolen’. I bet if the younger sis was still alive, the older sis would never be with the boyfriend anyway (even if he says he loves her). She loves her baby sis the most and would not bat an eye for anyone else. He got together with the older sis ‘cause he loves her, he chose her, not because she’s forcing him lol. Yeah they can be with someone else but that’s not how love works. The fact that they are together is NOT a betrayal! He does not belong to the younger sis (again he’s not an object, he has the right to choose to be with someone he loves). Main thing is, the younger sis has no right to stop them to be together. She’s about to die, she can’t control how the livings live. She was being very selfish and cruel, trying to ‘drag’ her sis to hell. How could you do that to you blood who loves you to death???

The same thing happened in "this is us" kind of. Its a show on netflix i this happens quite often when people die theres also a movie that has a similar vibe called "just go with it"

The most hurtful betrayal i saw is coming from the younger sister. For all the things her sister did for her, she chose a man who barely has any feeling for her. She took all her older sister's kindness for granted.

Haru one-sidedly loved her fiance and her fiance one-sidedly loved Natsumi. When Haru died, Touga was the one who took the chance to get together with Natsumi, so he wasn't stolen. Natsumi accepted still thinking about her dead sister but grew up to like Touga. Yes, it's kind of distasteful when a person dates the sibling of one you dated before. But you didn't even love the person you dated, so what's wrong with dating the one you like.

And I feel pity for Haru since the one she loved so much harbored feelings for her sister. Your sister didn't even love the guy yet you want to drag her with you. That's messed up. And Touga even said to Natsumi, that even if he liked her, he would still go through the relationship with Haru. Since as long as Haru is still alive, the sister wouldn't even bat an eye to Touga.
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I feel like Haru was too selfish. Yeah everyone's saying they feel sorry for Haru but I really don't. Natsumi loved Haru. Absolutely loved and adored her. All she ever cared about was Haru's happiness because she thought only Haru loved her. But what did Haru do? The sister she spent all her life with, she wanted that sister to go to hell with her even if that meant Natsumi and Touga to be separated. Natsumi hated Touga because he separated Haru from her. Still, she supported Haru and said she's happy if Haru is happy. But Haru never really loved Natsumi as much because she had many other people around her to love unlike Natsumi. She knew touga never really loved her. He was just being generous. But even so, she threw away her sister's love selfishly and to separate them, wanted a pic on her coffin, said Touga's name while dying. In my opinion, Haru was the most selfish and unlikeable character. Yes, Touga and Natsumi found solace in each other and it is immoral but you can't choose who you love. Haru had the love of her family and Touga's family, her sister devoted her life for Haru. But Haru cursed Natsumi for her petty jealousy. So in my head, Natsumi and Touga did nothing wrong. They brought happiness in each other's life. So they absolutely deserve each other.

I agree. With everything you said

I agree with everything you said.

I don't think it's a betrayal. They had respected the dead so much that they both suffered. The little sister was a victim of her fate but to say filthy things about her sister, whom always loved her, I think I would consider that a betrayal. Was it her sister's fault to be loved? The elder sister and the guy were decent enough people. They're not good but they're not bad either.

Lots of things I agree with, but two comments:
1. When I look back at the moment Natsume agrees to date Tougo, I'm getting a hint that Natsume is willing to date him, not just because she likes him, but because she is willing to having Haru's curse (the title of the series) on her, so that her sister would continue to follow her (during the last chapter she turns her head backwards checking to see if her sister was still there).
2. Frankly, almost all the blame should be put on Tougo. He's the one who convinced his parents to chose Natsume as his fiance after Haru died, and he's also the one who seduced Natsume, in that although she wasn't interested in him before, he provided her with comfort and literally saved her life during her worst moments after her sister died. He effectively became her pillar of support. So yeah if you wanted to blame someone it should be Tougo.