saying how she has different parents in the game, and is referred to as a friend and never a sister, seems like a good amount of evidence to say that she isn't a sister.
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Sorry for the delayed reply!
Back in 2018, Patreon updated their guidelines which didn't allow incest, so we changed Reika's previous backstory. You can read our official post about it here: https://www.patreon.com/posts/reika-change-17985850