As a regular NTR consumer, I agree that NTR doesn't quite fit here. The story is fine as it is, with the only challenge being that the protagonist has to avoid getting caught not being her brother and make his move. Could a new enemy come in to stir things up? Yes, that could work, but let's keep it NTR free.
Still, it might be due to a lack of support on Patreon. $620 isn't a very large backing, and with this move, they might be targeting the NTR audience. But I wouldn't recommend it based on the story and the existing "fanbase".