The true "secret" of the Parrs is that they are a normal family dealing with extraordinary circumstances.

The video ended. Violet sat in the lamp’s glow and felt the attic tilt under the weight of history. The camera’s waver was the same hand that had tucked the journal into the trunk.

3D computer graphics, loli, Violet Parr / Parr family secrets 2-8 - pixiv

Perhaps the most tragic secret of the Parr family is the psychological trauma inflicted by forced conformity. The family dynamics were highly fractured during their time in hiding:

On a rainy afternoon, an envelope arrived at the Parr house with no return address. Inside was a single postcard: a watercolor of a coastal town and a note in handwriting Violet recognized from the letters. It read simply: Jonah left. He was tired of hiding. Forgive me. E.