Reddit reviews I'm a Flower Girl! Activity and Sticker Book (Bloomsbury Activity Books)

That might possibly be the one book about parent relationships that hasn't been written. Typically a book in this area is written to help kids cope with a stressful situation, one they catch flack about from other people.

Is the daughter experiencing troubles? If not, you might try a more generic book like The Most Special Flower Girl: All the Best Things About Being in a Wedding or I'm a Flower Girl! Activity and Sticker Book.

I do not know if those books mention the nature of the bride and groom (e.g., whether they are friends of the girl's parents or not). I don't see Amazon's 'ask a question' option on those book product pages. You could try posting a comment on a recent review asking the reviewer if it's appropriate for a girl in a situation where it's her mom and dad getting married.