Maybe. I have always known what I wanted right from the age of 9. To work with computers. That was always my focus right through childhood. Nothing else mattered.
It was always computers, computers, computers.
I still don't think she knows what she wants to do.
Maybe that is a small part of it.
Even if it were it's no excuse. I don't really know what I would like to do career-wise but I've got jobs and I've kept them. For most of us, that's life. It sounds like your sister is avoiding 'real life', and fairly successfully as your parents have enabled her to, through parental love no doubt.
I've had help from my parents financially but I've only taken what's been offered, not asked. Nonetheless, while I don't regret doing so, it doesn't make me proud. It's also been on the proviso that anything given to me is also given to my only sibling, my sister (and vice-versa). I believe that to be fair and wouldn't have it any other way.
I'm sorry to say this to a stranger on the Internet about his family but your sister appears to be taking the P in the extreme. Almost limitless sponging at the age of 39. Your parents deserve better as to do what they've done thus far they must be caring, decent people.
Get this out in the open. There may well be fallout but you'll know that you were right in principle to do so. Even if the softly softly approach is adopted with your sister she may have confidence / lack of self-esteem / depressive issues that she could seek help for to enable her to focus on building her own self-sufficient life.
Not saying for certain that the softly softly approach is right in this case, but if you're following advice im this thread to the letter, there's only so many kicks in the minge she can take.