What sort of contact do you have with the GC? At what point do you accept they've evolved into an NSibling?

Brief history: I come from a family with a NMom and a EDad. My mother was everything in the book: alcoholic, drug abuser, borderline personality, verbally abusive... etc. Myself and 1 sister are in the process of reclaiming our lives. I am still in school and she has just graduated and about to start her clinicals in medicine. I'll be taking the national exams in my field in 1 year and will start working right after graduation (so excited!). We've both bonded over our journey of recovery.

Unfortunately, our last sister hasn't made any progress. She is horribly entitled and arrogant. I thought she would grow out of this phase but it appears as if her emotional and mental growth has been truncated from everything we've experienced. When we were younger, she was my mother's staunchest defender, almost to the point of being brainwashed by my mother.

Right now, she lives with my other sister (the one about to enter clinicals) and does not pay rent because the situation was originally meant to be temporary. Here are some things she does that really bother/disturb us:

• Completely oblivious to social cues (won't say good morning, ask people how they are, avoids social interactions, stares at people)
• Won't pay utilities or offer to purchase groceries
• Refers to herself as "highly intelligent" and tells us we aren't "on her level" (has been going on for years)
• Sleeps in until 12 every day (she is 24) and stays up until 2 am on the computer
• Eats other people's food and uses other people's possessions without asking
• Does not thank people, just takes what is offered and walks away
• Daydreams excessively while listening to music and pacing back and forth, daydreams during conversations with people and stops responding
• Frequently says she is busy and stressed out to the point of no function but sleeps 10 hours a night (I think she is paranoid about being sleep deprived)
• Leaves the oven and stove on, does not clean up after herself, leaves lights on, leaves heat on... generally oblivious to conserving energy and cleaning up after herself
• Refused to help babysit relatives (this was a big issue when my sister was getting ready to have a second baby)
• Turns conversations into arguments, extremely reactive and uncompromising, very dramatic
• Acts omnipotent and superior to myself and my sister
• Passive aggressively insulting of our intelligence and attacks us personally
• Has no friends and has had no romantic relationship ever

There are a million other little things that are hard to describe. I'm worried that she has become my mother.

If we confront her she then turns it around and says that we are wrong for "not accepting her as she is" and "not loving her" (classic NMom phrases). She calls herself an introvert and that's her reason for acting the way she does. My sister is planning on asking her to move out in order to shock her into behaving better. I agree because the situation is just getting worse as time goes on.

Has anyone else experienced something similar? If you have, did the relationship disintegrate eventually?

I feel like I'm at the end of my rope with her, but part of me still has hope. She's one year older than I am (we are both mid-20s), but I feel like I'm dealing with a 13-year old whenever I talk to her. Sometimes I think she has Asperger's or something, or at least she might be on the autistic scale. Should I start preparing for a LC relationship?

tl;dr: My sister (GC) is acting more and more like my NMom as time goes on. Has anyone else experienced this? Is it more effective to confront an NSibling than an NParent about getting help? How is your relationship with that person now?