She's BPD, nothing has to happen to you to make you that way...you just are. Some people are just completely self centered pieces of shit who live without any regard for the feelings of others.
She thrives on attention. She must always have constant attention.
From what I've gathered over hours of listening to her talk (I find her an extremely interesting case) she's had a totally normal, healthy upbringing. Small town, easy to outshine everyone else and get all the attention she craves. Then she went off to college...where she found out she wasn't as smart or talented as everyone always told her she was. She started doing porn because some Girls Gone Wild type film crew came to her campus.
There's nothing to indicate that anything that happened to her pre college had anything to do with her doing porn. The thing about her uncle stems from a stand up joke Geno Bisconti does regularly. He just points to a random woman and says "remember when your uncle said that", as a punch line. Scarlett simply took that, twisted it, inflated it, and milked it for attention. That's what she does. She lies. She makes up shit for attention.
I feel tremendously sorry for her family. Imagine the shame of having to live in a small town with such an utter embarrassment for a sister and daughter....but Delaney doesn't care, she never considers how her actions might effect others.
The look an Ralph's face on the BYB podcast was one of pure terror. She was screaming, like she does... banging her apartment floor with a hammer... without any consideration for neighbors that might be trying to sleep or that if one of them calls the cops a man she claims to love would end up in jail. She was even doing that crazy joker cackle right before Ralph ended the stream.