The little sister literally did nothing you have bad case of judging ppl. The kindest character in story had her first love stolen and she still cares for hoe of big sister. The egoism in first place prob happened after little sister got abandoned by the guy. He changed love like pants.
You will probably push away kind ppl irl blaming them they are pushing away be careful lmao
Wait, the big sister is the one that was the first girlfriend of MC, that is serious and tries to be considerate of her sister and the MC, no? And the little sister is the smart one that's aloof and as much as she wishes for the happiness of her sister and actually ended up liking MC, still goes extremely "my pace" and barely tried to meet halfway with him? or am I getting the sisters confused?
Edit: OK, just speed-read through everything once more, and though I can admit that maybe I was wording it very strongly, if not maybe exagerating a few events I vaguely remembered...I still cannot think that Naori (the younger sister) is particularly kind, least of all purely innocent. Sure, we're shown a few times that she seems to mind other people, but most of her monologue is self-centered, and almost never in a self-sacrificial way. She thinks mostly in terms of "having her cake", when she has barely tried to exert herself for it. The one time she actually sees herself in a negative way for how she acts was because she couldn't actually confess to the MC before her sister did. And she relishes in the fact that her sister decided to let both of them be together, even while they were happy before, because it means now she can actually have her love. She barely gives a thought about her sister doing it while obviously hurting about it.
Like yes, it is kinda sad that things happened that way, but with the context of how she was acting throughout it...I can't say I feel particularly sorry for her, when she already had all the advantage needed and basically acted like she didn't need to worry about actually getting what she wanted, and the only reason she feels sorry is because she slightly regrets actually worrying over the change in relationship more than she should've and ended up not getting what she wanted because of it. At least compared to Rumi (older sister), which worried a lot more about it all and actually pushed herself to see what would come from it, while thinking she didn't actually have a chance.
Regardless, the MC certainly is worse than either of them being indecisive with dedicating himself to Naori now, though from how Rumi basically pushed him into having to deal with all that out of nowhere, it's kind of understandable. Like, the one reason I'm not being as harsh with Rumi than I should be (she ended up regretting actually getting her love and decided to let him have what she thought was the love he wanted in the first place, regardless of his feeling at that moment) is because she seems to just naturally value herself so little that it feels sadder to see her trying to give up on the things she managed to actually get with her effort. Just like she exerted herself to confess to the MC, and seemingly actually put in enough effort that he fully reciprocates her feelings, she's exerting herself to give up on him.
In short, I cannot see Naori putting any significant effort into the relationship, compared to Rumi.