The PC married these girls' mother, so he is not directly related to them. BUT! There are 3somes and whatnot that have both girls together, and sister-sister is still incest, hence the tag.
Also, really good game that turned into a decent hentai-anime (with both parts out at the time of this writing.)
Edit: I almost forgot, the mind-control / mind-break comes into play with the drug he uses, which is pictured on picture 170. Don't know much about it, but I do know he utilizes it in some way throughout the story, and it has awesome-sauce powers (that makes me want the drug myself...)