Lily was always bright, in all senses. She was kind and smart, and even charmed that dirty rat (who was the first who introduced the title Petunia will be called by the rest of her life - "a muggle"). Everyone liked Lily, so she must be a heroine, who will find her own prince and live happily ever after. And Petunia will protect and support her. Because that is what an older sister should do. Even when sisters argue they will get back together, as there is no one closer in the whole world. That's what her mother said.