About Haru, and I know I am blabling a lot

It's different for everyone but I think Haru's blog and her plot add tons of realistic details into the plot.
To me, for Haru, despite being thought of by Tougo's family as the better option, she herself is insecure. She is quite a meek girl who believes that despite what others said, she is actually the inferior one compares to her sister (as can be known through the scenes of her bragging about her sister, even when going through the turmoil of knowing Tougo's feelings for Natsumi).
And as she is immature and given the condition that she didn't have much time left, things weren't going well for her. That is where it got ugly. She didn't have the time to mature and accept the reality that the one she loves (Tougo) loves someone else and that someone else is the source of her insecurity- her sister Natsumi, instead, she was plagued with pain, which got worse with each day gone by, so simply said her mentality took a slip for the worst and all the nasty came forth like waves to the forefront of her mind, every single day (cause her sister visit every.single.day). Her inferiority complex fueled her nasty thoughts on her sister and the more she thought of her sister in that way, the more insecure she became, thus began a vicious circle that gotten way worse than when it began and only ends when her life ends.
You could say writing that blog was a source of relief for her for if she let out her emotion there, the less damage she would do on both her sister and herself (because even if she hates her, she still care for her, even if in the end it became twisted).
So yeah, in general, I don't think she that bad, I think that if she were given time, maybe she would have just learnt to accept it or continue being ignorant for a while (possibly even after the marriage, drama, tons of drama, then finally learn to accept and let go of the situation, etc). But if she had lived, I can only imagine that her sister would probably try to kill herself more than just that much that was shown in the story if she knows that Tougo loves her. Shit, still, that drama could have been fun and tear-jerking too.
On the contrary, there's the possibility that things wouldn't even go well if she were ever able to survive, but that is not happening because...in the end, she's gone.