oh, i never got this item, what does it do?
Seen there is no description to it (not even the ".................." from EP fragments) was it really meant to be in the game?
That's what we're trying to figure out, MakatoYuki90 said it was in the cloud level before the boss, but i think he was probably thinking about the same heart you were.
I think at one point in development, maybe an earlier build, the heart item existed to make the sister allow you to have sex with her in the non-EP world, but i don't know for certain. I think Foyuu just didn't take it out of the game files when it was no longer needed and just took it out of the level it was in, which is easier and faster.
OR maybe it needs to be in the game to trigger something, its just not meant to be in our inventory. I don't know. Without unpacking the game and messing with the files in RPG maker, i can't say, and i only have RPGMaker MV, don't read Japanese, and have no idea which, if any, mods he used for the game. So i'm stuck with deductive reasoning.