The Dursley Witch (Songbird Curse)

Dudley married a woman named Sally. Sally had a secret.....

Sally's sister was a Witch.
Dudley never told her Harry was a Wizard.
Sally's family was cursed! If she where to conceive a child with a Wiz/Muggle ( A muggle with a history of family Wizardry even if it was multiple generations before.) She would give birth to twins one whom would be a Wizard.

Sally thought she would never have the curse in her family, Dudley seemed normal.

But The Dursley's are thrown by the arrival of Billie's Hogwart's letter.