Twins of light and dark

Since birth, Dipper and Mabel Pines have been hiding something from their family, apart from their parents, and they discover each other's secrets one night at the Mystery Shack in Gravity Falls.
In fact, Dipper and Mabel are not human; Dipper is a creature of the darkness, a vampire to be exact. Plus Mabel, his twin, is a creature of the light, a fairy.
Not only this but their parents are not entirely human either.
Ford and Stan grow suspicious of the twins, convinced they're hiding something, and go to find out their great niece and nephew's secret so they can find a way to protect them.
(this is based on a RP I once did. Hope you like it :D)
(I don't own Gravity Falls, only the plot to this story)