The founders and Merlin himself are not only some of the most powerful witches and wizards of the age , creators of the best magical school there has ever been and evolution of magic for centuries to come. They are also some of the most powerful, dangerous, captivating, and gorgeous mythical creatures in existence not known in history that just goes over their amazing feats and magical potential. When a very powerful witch or wizard turns 18 they go into a mystical creature inheritance and are either a dominant or submissive the more powerful of a magical creature you are the the more likely you will have more then one mate, what is not known in history is Godric Gryffindor, Salazar Slytherin, Helga Hufflepuff, Rowena Ravenclaw, and Myrddin Emrys known as Merlin were all bisexual and mates and balanced each other out and after hunting for their last three mates for a long time , since they are so powerful individually and needing at least 6 or 7 life bonds finding one, they get a present from magic herself , a book from far in the future detailing the lives of their two mates.
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.