WMG / Hemlock Grove

Shelley is a dhampir.
Before I explain, I'll say that this is entirely based off of information available from the series, as I've yet to start the novel. Now, not a lot of information about Shelley's past is presented in the series, but what we do know is that she, like most of Olivia's other children who are not Roman, died shortly after her birth, and that Olivia didn't mourn her death because she was born without a caul, and thus, wasn't Upir. We also know that she was brought back to life through Pryce's research, and this seems to be what led her to have glowing skin. Beyond that, we know nothing about whether her physical appearance and inability to speak were preexisting conditions, or side-effects of the process that brought her back to life. Based on the specific types of mythology included on the show (i.e. vampires being born with a caul, or werewolves being created by drinking water from a wolf's pawprint), it seems likely that Shelley is intended to be a dhampir. Most dhampirs that appear in current media tend to be more of the "basically a vampire but without all the weaknesses" variety, whereas a good portion of folkloric dhampirs were deformed beings who would die very soon after they'd been born because, as the child of a vampire, they were already half dead to begin with. The reason Olivia treats Shelley so poorly may have less to do with her not being upir like Olivia's golden boy Roman, and more to do with the fact that, in folklore, dhampirs who do make it to adulthood are said to have great skill when it comes to dispatching vampires and (occasionally) other creatures, like, for example, the werewolf that Shelley kills with her bare hands at the beginning of episode 13.

Shelley is similar to Frankenstein's Monster
Particularly the novel version of the creature. Shelley is large, malformed, and has been through multiple surgeries to correct the deformities. Her name could be a reference to Mary Shelley.

killed her parents She lives with her grandparents, and given how much of a psycho she proves to be, it's not that farfetched to think she was responsible for the death of her birth parents.

will be Back from the Dead in some form. Peter had to cut off his grandfather's head at his funeral because he said "bad things happen to my kind if you don't cut off their heads." And Christina's body was taken by Sheriff Sworn before anyone could cut off the head, so that "bad thing" is certainly going to happen to Christina.

Peter and Destiny will enter a romantic relationship with one another.
The show hasn't shied away from incest so far, and they seem far fonder of one another than they have any of their love interests, beyond what is even typical of cousins.