Voldemort decides to take Harry in as a son the night he goes to kill him. Without Harry defeating Voldemort when he was younger, Voldemort continues to increase his power until he takes over Britain.
People should have known that Voldemort's ambition would not stop at Britain. With his sights set on Europe, Voldemort tries a new set of tactics, politics. Grindewald tried war and failed, why should he expect anything different.
Enter Thomas Peverell, the newest politician, who loves to drag his son Hadrian to all of the richest parties on the continent to rub elbows with others of power.
It was at one of these parties in France, bored out of his mind, that he sees her. She is in a similar situation having been drug to many of these parties since she was a little girl. She is tired, she has already had to turn down 15 people tonight, and then she sees him.