I’m not an expert at all and i’m under 18 so i am not diagnosed with BPD but I show symptoms of it and have been through DBT which is used for bpd so I know quite a few handy things for regulating emotions.
Firstly, it sounds like you love your sister a lot which is great, it’s good you’re willing to help her as much as you can. Let her know how much you love her as much as possible. i’m not sure as to what her symptoms are but i’m aware that a common one is feeling people are against you and that’s sometimes even family. try to respect her confidentiality if she confides with you unless she admits to feeling suicidal or is in any form of danger. Sometimes if you feel you can’t trust people then finding out they have openly shared sensitive conversations you’ve had can confirm these suspicions.
Secondly, in an episode (a time where her symptoms are showing ie. panic attacks, hysterics or delusions) it can be very scary for you in the moment and no doubtedly it is for her too. DBT is a therapy that helps people with these moments and i suggest it’s worth having a rough understanding of. Some of the techniques for calming down in these moments are used in acronyms to help people remember them; STOPP, DEARMAN, TIPP are just a few and are worth googling. learning these could help you talk her through episodes. I have these acronyms written out on flash cards to help me remember them and I keep them in my bag for emergencies. maybe open a conversation with the family about learning along with her if you feel you can.
Lastly, as a sibling you are not responsible for her at all and don’t blame yourself for any behaviour she expresses because of her illness. The fact that you care enough to ask is alone enough, and as long as you try your best to help her when you can you’re doing your best which is all anyone should expect from you. Remember to look after yourself as I know from personal experience it’s not easy living with a sibling with mental illness. As for self harming is anyone else aware of this? if she’s continuing to do so or you find she relapses it is completely justified to break confidentiality and tell an adult you trust and please do no matter how much you think she doesn’t want you to. she will forgive you.