Petunia was just jealous. She wrote a letter to Dumbledore asking if she could attend Hogwarts with her sister and was rejected in the basis she was a muggle. She held scorn for the magical world from that day on and hated seeing her sister do things she couldn’t and her parents were excited by. So she told her husband of it’s existancd and called them all “freaks” for the rest of her life. In truth she probably loved her sister, but never got a chance to make ammends. After all, her sister died shortly after she got out of Hogwarts. 21 or 22