Rachel and Theo are fraternal twins with a very close bond (normally they share a psychic link), but following the sudden death of their mother, Theo has pulled away from Rachael. Staying with family out of town, Rachel and Theo attract the notice of their creepy neighbors, the Wilberforces. They quickly realize that they are being hunted, and by beings who cannot possibly be human.
They meet Mr Jones (Sam Neil), a long-lived member of an alien race, who has long battled the Wilberforces. Mr Jones explains that he comes from a race of twins who always share a deep bond, and that his twin died when he journeyed here. He has been searching ever since for other twins who have the same strength of "twinness" so that he can give them the powers (he is a fire-raiser) they will need to defeat the Wilberforces (who are a destructive race that consume other worlds).
Theo continues to resist his bond with Rachel, thus weakening their abilities to wield fire. However, he eventually relents and they are able to use their powers to destroy the Wilberforces and save the world. Mr Jones dies helping them.
Thanks Mary C!