My advice would be to help your sister by removing her from her current environment and putting her in an evnironment where she will learn to be healthy and emotionally stable and be inspired and motivated to be someone who is healthy and emotionally stable.
I think that people who have mental health diagnoses tend not to get better for as long as they remain in the mental health system. It's like a self fulfillig prophecy and worse still, those who work in mental health, psychiatrist and clinical psychologist have the terrible attitude that these people cannot be fixed. What is more, those with mental health diagnoses such as borderline personality disorder are even rewarded for their behaviour by recieving welfare benefits and it become easier to just stay on that rather than work and even on a subconcious level people can feign symptoms at the slightest hint of their benefits being withdrawn.
Of course, your sister is suffering at some level and needs help, I don't deny that, but the environment she is in is probably perpetuating it and may well have created it, usually it is the evinroment and stressful relationships that causes BPD. These things are usually created by family relationship complications and often there has been some kind of abuse to some degree somewhere along the line.
I think that if she were to leave your family house and stay with one or two people who she really admires and respects, who are really nice, caring and good role models and who inspire her with a new interest, a passion that she finds she would like to pursue along with also giving back in the form of volunteering to help other people, then she would be so focused on what she is doing that she would naturally feel better in herself and she would also develop better ways of dealing with people along the way.
I think it is important she does this completely away from her family. She needs to space to learn, grow and develop without her family watching, without embarrassment and without her thinking about her family or original environment. She needs to be in a completely new and fresh environment and be inspired by healthy people she admires. There's probably no chance this will happen in her current evinroment.
If she were to remain in her current environment her behaviour will probably continue, it will effect the health of you all, she will spend her life like this until she gets to about 45 years old when she will probably start to mellow out a bit, but then she will have wasted so many years where she could have been healthy and done good things.
An interest and education:
Ok. So how to find such an evironment for her. Think about anything she likes doing, anything that makes her happy. If you can't think of much, then try and find something by leaving magazines around with these topics. It could be travel, bikes, flowers, history, sport, art, design. Then look for some kind of place she can go to pursue this further and look for people who would be good mentors she would admire. It is important for her to be around mentally healthy people who she doesn't feel intimidated by and made to feel inferior. In time she will pick up on their habits as long as she doens't have a negative attitude to block her learning and lowering her self esteem and causing her emotional problems.
Helping other people:
Try and find something she could do that would help other people. This would be good for her self esteem and self esteem really improves people's behaviour.
Sport:
Try and get her involved in physical activity. If she has an exercise routine she will feel happier. The best thing would be to join a local fittness group in the park where she can run around in a team doing exercises in the fresh air. She will certainly feel happy after doing that. Of course she'll ache the first few times, but she'll get fit.
I think that people in reciept of mental health services should volunteer and help other people so that they don't get stuck in a 'helpless patient recieving help and services mode'. It is really empowering to be in a position where the patient themselves are the ones helping and giving care to people. When working as a helper for others, it changes their self perception of themselves and improves their self esteem so much.
Doing this can literally cure a person of so called 'borderline personaltiy disorder'. If the person remains in their current evnironment and continues to live this psychiactric identity which defines them then they will never get better and will continue to drag you down.
Areas to avoid are those where alcohol and drugs are often the culture or consumed. She should stay away from that.
Hope that helps.