From what I can tell from the story so far the girls older sister made a wish on a supernatural item that made her wish come true but sacrificed her most important thing. We'll have to wait and see what the deal is with that
Lead female is cute, MC is interesting, art and story are good. I see no reason to not read this. I mean, I WANT to read this! That first chapter was good! But that damn tragedy tag...I just hope it's got something to do with that Star Tear thing and his past rather than what's to come...
There's a lot of hints that something terrible happened to the MC in the past, but I cant tell if it is supernatural or not. So far, it seems it is supernatural, but no the "power up" kind, but the "much cost, much hardship" kind
I think supernatural tag more fitted then fantasy tag. As for tragedy tag, what i got from web novel teaser volume 1 (full story only found on Light Novel version)
For ryuuki
futahara ryuuki actually killed in accident
For touka
futahara touka its actually futahara ryuuki itself
Last for my speculation
Iori who despaired for ryuuki accident wish to save the life of his childhood friend make wish at meteor shower like in the 1st chapter at the cost losing his emotion & memory of ryuuki (it replaced touka/ryuuki memory as ryuuki little sister) that's why the title this manga "I'm still a "my childhood friend's sister""