Hi @Flealee ...
Wow, it sounds like a really tough situation...I can understand how this would be taking a heavy toll on you and burning you out...
I can empathise as I am in a similar-but-different situation where my brother is experiencing mental illness but will not seek help no matter how much we all encourage him to. At the end of the day, I have to accept that it is not my role to be his therapist and I cant force him to do anything. Its out of my control...much like how it is for you and your sister. You've tried your hardest to help her, but she is refusing to take any responsibility to help herself. You are not her therapist, and its not fair for you to be burnt out and suffering due to her behaviour. It is very difficult to accept something is outside our control, especially if it involves someone we love. We cannot change anyone, or help anyone to change, unless they want to change themselves - taking responsibility for ourselves is paramount.
I would encourage you to do what you need to do to protect yourself. It might help to remember that "you cannot pour from an empty cup"...
If you feel you have the strength at the moment or some time in the future, and would like to try and maintain communication with your sister, remember that setting boundaries is absolutely vital (and a core component of treatment for BPD)...it is protective for you as well as for them. As such, it may be helpful to clearly outline what your boundaries are to your sister, as well as the consequences of breaching them, and stick to this without wavering...advising her that you love and care for her, but A, B and C are behaviours that you will not accept, and that should she engage in these behaviours, the consequences will be X. And make sure you stick to what you say, even though it is hard...you will know what an appropriate consequence will be (e.g. "if you are abusive towards me, i will end this conversation...though i will be available to chat again next week and will call you on Monday")...make sure you stick to what you say; predictability is key. The point of ending the conversation but offering a call again in a week says to your sister that you will not accept bad behaviour but you are not abandoning or rejecting her. While her behaviour is bad, SHE is not bad. You can have a disagreement with someone and this does not mean its the end of the relationship or that they hate you. Over time and with repetition, hopefully this will assist in your sister respecting you and your boundaries.
Sorry for rambling on. I hope this isnt too confusing and is of help.
Just remember that you absolutely are not her therapist and do not have any responsibility or obligation to "fix" her. That is not your job. Your only job in life is to keep yourself healthy and safe. You cannot pour from an empty cup.