Voldemort gets it right, everyone else gets it wrong

Dumbledore is dumb-as-a-door and the potters believe him. Helena Potter is the Girl who Lived but everyone thinks it is her brother Ryan. She gets foisted on the Dursleys then the founders rescue her and bring her up.

Disclaimer: Jk Rowling owns the HP universe not me, all I own is the plot. Credit to Fantasticoncer for the character of Helena, who I based Helena Slytherin-Hufflepuff on.