I have had a very tumultuous relationship with my sister for our entire lives. We are very different people. She is very selfish on many levels, and likely could be diagnosed with some mental disorder. Earlier this year, I texed her, asking her to make a paper sign saying "Happy Birthday" to my son, and send a photo of her holding it. My wife and I collected dozens of photos from family and friends, which my son loved. My sister was the only person who didn't respond. She ghosted me. My mom told me she said she doesn't like taking photos of herself, which is why she ghosted me. My sister recently used AI to write a book, which she self-published. She texted me recently, asking me to buy a 99 cent Kindle copy, and leave a positive review. I don't have a Kindle, nor do I want to buy the book. I haven't written her back yet, but likely will later today. AITA for not buying her book?