"Never turn your back on a Hastings." How many times have the Hastings family been called out for their manipulative, secretive nature? Too many times to count. Even Melissa (Torrey DeVitto) told Spencer that she had been "protecting" her "since the beginning." Could she be referring to Spencer's evil twin, lurking in the dark shadows, waiting to harm her big sis?
There's also the fact that Spencer has a history of drug abuse, and has occasionally blacked out and forgotten pretty important memories. Veronica even tells Spencer that, during Spencer's darkest times, she wasn't sure if it would be Spencer or her "evil twin" coming down for breakfast. Well... could that be because Spencer has a literal evil twin? If so, perhaps Spencer's worst behavior wasn't actually done by Spencer at all, but by a twin hoping to mess things up for the girl who got to live out her perfect life.