Unless there has been significant trauma in a person’s childhood, the way we turn out is ultimately up to us. I don’t think the lack of a sleep-over qualifies as a permanently damaging event. Most parents do the best they can with what they know at the time. Inevitably, they make mistakes. Inevitably they do and don’t do things that in 20-20 hindsight, they wish they had known to do differently. Usually their adult kids do let them know of their “failures” as well as their successes. This is usual life.
It’s so much easier to blame others and to talk about what they should do or should have done rather than to take responsibility for ourselves. Even if all her stories were true (which you say they are not), ultimately it doesn’t matter. Your sister is now an adult woman in her 20s with a college degree. It’s up to her to do the hard work of making her life what she wants it to be, rather than blaming others for what hasn’t been given to her.
It’s possible that you sister has a significant mental illness. It’s also possible that her lies and fault-finding are the result of deep insecurity. In order to shore herself up, she has to put others down. She may be convinced that she will fail if she takes responsibility for creating her own adult life. If so, a way out of doing it is to blame others for how stuck she is.
Like many people who are scared and defensive, she may tell her lies to herself with so much conviction that she ends up believing them. Such people are very difficult to help because it becomes almost impossible to argue them out of their positions. Facts don’t convince them. Rational argument doesn’t win over someone who is being irrational.
Ideally, she would get some therapy. But she’s unlikely to do so since, from her point of view, she’s fine. To her, the rest of you own the problem. I think the family might find it helpful to talk to a family therapist about how to navigate this painful situation. A therapist can help you learn some strategies for managing her behavior and for protecting your own feelings in the process.
I wish you well