4. The Reasoning for the Twins Theory
This chapter is by Ahlaam Nightshade.
Coming along from what Prez said about the twins, I have a theory that Bill Cipher entered into Dipper and Mabel's parents' dreams, and then persuaded them to send the twins to Gravity Falls. This is because Dipper and Mabel, being the Great-Niece and -Nephew of Stan (and possibly Stanley), are the only ones who can stop him from gaining the Journals and taking control.
Bill lured the twins into Gravity Falls so that he could kill them.