"...There was no point in worrying Mrs Dursley, she always got so upset at any mention of her sister. He didn't blame her - if he'd had a sister like that... but all the same, those people in cloaks..."
I can't understand why the structure "if he'd had" is used instead of "if he had" here. To my understanding, this sentence is describing Mr. Dursley's thought in an indirect form rather than the author's comment.