I feel like they made that up, to test her, and make her show her true color. I also don't understand why the fake sister is trying that method to gaing more power. If it was me, I would try to get closer to family members, while the OG sister was out of the house, and then wait to find dirt to later. Because now if it comes out, that he is allergy to purple, then she can't blame OG sister, because she was gone when they found out that info.