I wanted to write a review with a neutral tone, but i can't get myself to do it. I'll be harsh and won't talk about what i liked for the sake of constructive contradiction (who am i kidding here ?). My apologies, as this manga still has its good part(s) and could have been a lot worse.
The obvious and admittedly completely subjective reason for my disappointement is that i hated this onii-chan's character right from the start. First he's waaaaaaaaaay too perfect (over-considerate of his sister's well-being, not to mention he's over-beautiful, over-skilled and over-wealthy on the surface) ; When i can't help but picture a guy as Richard Gere, it's usually really bad omen for my entertainment. Then, throughout most of the show, his patriarchal decisions don't feel natural to me within the context i'm given to behold ; i'm not saying he's stupid or unreasonable for doing this or that (this is what drama is all about), but that more often than not i found his choices simply inconsistent with his objective love for his sister and his objective extraordinary abilities.
The same facepalmgenous decision making pattern goes for his sister's behaviour, but fortunately not as often (i'm probably more benevolent). And seriously, who put into the author's mind that buckets of tears equals effective dramaturgy ?
I won't even mention most of the supporting characters (the exceptions being the classmates) that are only thrown into the setting to fulfil a specific task and absolutely nothing more.
Storywise, i kind of enjoyed it as long as there is indecision in the sentimental outcome and it doesn't take itself too seriously (in spite of the dark topics involved), ie. roughly until halfway through. After that, it just becomes one of these wannabe tragedies that i'm not personnally fond of : every single element on the face of Earth seems to be intriguing against the protagonists' living happily ever after, gods, nature, family and foes alike. Please, modern Japan is not mythological Greece.
More depth in the characters and situations would have been welcome in my opinion, instead of groundless and forced "oh, i'm the antagonist so i'm gonna do something real nasty to them", "oh, i love him/her so why don't we just forget about each other". The way the whole truth is progressively uncovered felt awkward and unoriginal to say the least. But by then i had already lost any hope and interest.