How to Get Away with Murder: What Happened Between Hannah and Sam Keating?

[Warning: The following contains spoilers from Thursday's episode of How to Get Away with Murder. Read at your own risk!]

Just when you thought we were solving How to Get Away with Murder's final mysteries, another one pops up out of nowhere! Instead of asking ourselves whether Annalise (Viola Davis) will get away with the group's many murders, we now find ourselves asking an even worse question: What happened between Hannah and Sam?

Thursday's episode, titled, "Let's Hurt Him" saw the death of Xavier Castillo (Gerardo Celasco) at Nate's (Billy Brown) hands, which ironically was only the third most shocking twist of the night. Not only did Frank (Charlie Weber) seem to completely wash his hands of his star-crossed romance with Laurel (Karla Souza) -- finally -- Sam's first wife Vivian (Marsha Stephanie Blake) showed up to throw another wrench into Annalise's plan by digging up old family drama of the Keating variety.

In an effort to protect Gabriel (Rome Flynn) from being called out as the "anonymous witness" who saw that FBI agent kill Asher (Matt McGorry), Vivian offered to tell Annalise who's really been gunning for her and pushing this investigation. That's right, it's not the Castillos, or the FBI, or even the governor that launched this attack on Annalise. It turns out, it was Hannah Keating (Marcia Gay Harden), Sam's (Tom Verica) sister.

That didn't seem to shock Annalise since Hannah spent most of Season 1 trying to prove that Annalise had killed Sam, and the grudge between them made Laurel's turbulent relationship with her family seem tame. What gave Annalise pause, however, were Vivian's final words. "You don't know... what happened between her and Sam."

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When TV Guide spoke with Pete Nowalk about the final run of episodes for How to Get Away with Murder, he teased that one more big twist was on the way, and we would never see it coming. Call us crazy, but learning some deep dark secret about Hannah and Sam's past seems to fit that bill.

Now, we hate to jump to conclusions, but -- how do we put this delicately? Hannah always seemed a little attached to her brother. Annalise even accused her of incest with Sam when things got particularly tense during the initial investigation into his murder. What's worse, Hannah got a sour look on face, but she never denied it or even lashed out in disgust. Silent sulking isn't exactly the reaction we'd expect when someone accuses you of sleeping with your brother.

Vivian didn't have anything good to say about Hannah or the Keating family in this episode either. She warned Gabriel off of meeting Hannah, telling him she ran away from the Keating family for a reason. She has always been vague about the end of her and Sam's marriage, but we just assumed it was because he'd cheated with Annalise. Vivian confirmed tonight that Hannah was part of the reason she left too.

And doesn't that just make the mind race?

Incest definitely seems to be the most likely explanation for this hidden secret about Hannah and Sam's past, but we're not ruling anything out at this point. When it comes to How to Get Away with Murder, the minute you think you've got everything figured out is the moment you should prepare to have the rug pulled out from under you.

How to Get Away with Murder airs Thursdays at 10/9c on ABC.