...I'm going to get dragged into all of my sister's problems...
No, you're not getting dragged into all of your sister's problems, you are letting yourself get dragged into all of your sister's problems.
And if I don't help them, her life is ruined and I'm overwhelmed with guilt for my whole life at not helping the one person in life I'm closest to.
You cannot help her.
Do you know the difference between enabling and empowering? What you have been doing all your life is enabling your sister to manipulate you and everyone else via guilt trip. She might not do that intentionally, or even if she is aware of the burden she puts on your shoulders, she is not able to alter her behaviour (without professional help at least). That's borderline personality for ya.
As long as you keep cleaning up her mess and picking up the broken pieces after her, you are actually not helping her, it will just reinforce her behaviour.
EDIT: I wish I could give you advise that would fix it all, but sadly I can't.
I can give you some basic advise though.
1) Set boundaries on what is acceptable behaviour and what is not. She cannot do that for herself, so that is up to those close to her.
You will have to learn to tell her 'no, I can not do that for you'. And 'no, you can not expect that from me'. Expect tantrums, and emotional blackmail remarks like 'you are an egocentric bastard', and ' you dont care for me at all'. Don't give in to those.
Tell her you love her, but that she is crossing the line of what you can and cannot do for her.
2) tell her excactly how her behaviour makes you feel when she does something that makes you feel shitty or guilty. After things cool down a bit, not in the heat of the moment, but don't wait too long.
Example: if she made a remark like 'you don't care for me at all' in a tantrum, when things cool down, sit down with her and tell her 'do you know how it makes me feel when you tell me I don't care for me at all? It makes me really sad / angry / desperate, because I do care, a lot'
3) Never push her to get professional help. She needs to get that idea by herself. Ofcourse, you can hint at it, but never in a 'you must do that' way. If she ever does show any interest in help, be supportive.
EDIT2: Also know that there are both self-help groups as well as professional councelling for people close to people with a borderline personality disorder. At least over here there are.