The sister
Our story starts in a car outside of a fish and chip shop. Petunia is on a date with Vernon.
Petunia delicately stated, "Vernon, dear, there's something that I have been meaning to tell you about my sister."
Vernon grunted. "You mean Lola or whatever her name is?"
Petunia corrected him, "Lily."
Vernon said, "Yes, that's it."
Petunia told him, "I told you once before that she's a freak and so is her boyfriend, but I never quite explained what I meant by that. She's a... witch."
Vernon chuckled. "Very funny, darling."
Petunia frowned. "I'm not joking. She's a real witch, even though neither of our parents have any magical abilities."
Vernon looked at her and reassured her, "Ah, well it could be worse. I won't hold it against you."
Petunia asked, "You won't?"
Vernon replied, "Of course not. It isn't your fault that your sister and her boyfriend are freaks."