I think one of the tags listed above may be in error. The "protagonists full sister as a heroine" tag makes no sense. The games description lists the protagonist as an illegitimate child between a dark-elf father and a human mother, inherently resulting in him being a half-elf while, based on what I gleaned from vndb.org, his three sisters all appear to be full blooded elves, so he obviously doesn't share the same mother with them. Unless there is a fourth heroine I'm not aware of, none of the heroines can be a "full sister".