Dot Kobo - Crystal Story Completed

As far as I could understand, it's the usual "Heroes vs. Demons" kind of story, in which can end with bad endings for the.. female companions? For your sister? Since in the beginning our protagonist gets called "Onii-chan" - unless it's not the true, but another "kind of". But either way, is that kind of game that you can rescue and then bad things happens after a set amount of "time" if you don't hurry, I suppose.

Guess that some NTR is in it.
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I mean, if someone have a better overview, feel free to share it. Having google as helper.. it's not that great.