Nice one Hannah

I think it's the second girl, because by saying first girl is lying (a true statement), she would no longer always be lying. So this means Carol must be lying. In fact, her name must not even be Carol, since she always lies. Which means, you can't even trust her when she says that at least two of them always lie.

hmm.. I think this means you can't trust Carol's math either. Thus at this point perhaps we can even assume that "at least two of us" can mean anything. Switch that to the reality of "at least one of us (being Carol herself)" and now we have a fitting scenario.. I think. That, or my brain just decided to stop chasing its tail.

What about the first girl's lie? Still a lie, but a coincidental one unrelated to Carol's statement (as Carol's statement can't be true). So while the first girl was lying in this instance, she is not prone to be always lying. Perhaps she even knows the second girl, which would explain the "ignore that bitch" intro. Two friends playfully interacting with each other. (and it is hypothetically possible that she knows this girl while still being your long lost sister. They may have been introduced to each other some time before the start of the reunion)

Because Hannah figured if the guy came to her mother's funeral he'd come to her sister's. So she'd be able to see him again.

Except if a psychopath puts themselves above everyone else, and has an iq over 80, they will realize that by killing someone they put themselves in jeopardy.
Ex: Murder with a flaw > get caught > jail.
So even if the girl was disposable, unless the psychopath was sure they could create a perfect murder situation (which generally takes a year of planning), the psychopath would not kill her for their own sake.

Basically Hannah is a special olympics psychopath.

Here's one for you. A boy is given a bicycle and a football for his birthday, but he starts crying. Why is he crying?

Yup.