I don't remember a grave, but I do remember their father speaking as if he had two daughters.

Well, could there have been two daughters but one died in such a traumatic event (trauma to both) that the split that developed in Nikki's personality took on the persona of her sister?

The fact that Jessica actually existed doesn't exclude the possibility that Niki has a split personality.

Yeah, they had the dad tell Jessica that she was dead, and Niki went to her grave. But Dana's right that this doesn't negate the split personality theory. I think they're going with the split personality as a power, though, what with Peter talking about how much he wanted to Darth Vader the Irish guy to death and then camera showing the godsend tattoo disappearing from his arm.

There were two daughters. One died, at the hands of their father's abuse, and the other developed a multiple personality -- which happens to have the same name as her sister, although the real Jessica was young when she died, so it's doubtful the personalities are similar at this point.

Adopting the persona of a dead sibling -- one who was seen as a "protector" -- strikes me as not unreasonable for a personality that emerges to "protect" the real person. Indeed, without looking it up, I'd be amazed if it's unprecedented.

Indeed, without looking it up, I'd be amazed if it's unprecedented.

Agreed - it practically feels trope-ish to me.

i'm not negating that Niki has multiple personality disorder. just that Niki and Jessica weren't always one and the same, which is how i understood Jon's post.

the writers definitely need to spell out her powers a bit more specifically if we're all still this confused about them though...

I think they're going with the split personality as a power, though

I'll admit there's something up with the tattoo, but I sincerely doubt that the MPD is in any way a manifestation of Nikki's powers. The fact that she can have them when Jessica's not driving seems to give credence to the idea that they're not directly tied to the personality, and she seems to be able to turn them on and off (meaning she's not super strong all the time. She has to make a conscious -- or at least subconscious -- effort to become strong.) The MPD is a trait of her character, and -- as is often the case with MPDs -- it probably kept her alive through adversity, but ultimately it's a tool that the whole person needs to discard, because it's outlived its udefulness and become destructive.

Peter's situation is different. He's probably got a lot of emotional damage going on, which continues to exist, amnesia or no. When he was given a chance to lash out, he did. And has probably needed to since he blew up.

Victor's theory requires a bit of nuance on the part of the writers, which isn't really the show's forte.

i'm not negating that Niki has multiple personality disorder. just that Niki and Jessica weren't always one and the same, which is how i understood Jon's post.

I should have clarified, sorry. Like everyone else has said, there was a real Jessica. Another Jessica. But there's no "possession" or other supernatural/superpowered thing going on. The "Jessica" that is/was within Niki, is NOT the same Jessica that was her sister. It's just a manifestation of her psychosis.