Hi WindyDay
Dealing with one disordered family-member can be difficult enough, let alone two
I have a father that was diagnosed in the late 80's. Mind you the only reason he actually got help was because he stabbed himself at work and was pretty much forced to.
Quite disturbing that your dad did this. Is he still getting help now?
Good that you've been able to establish good boundaries with him though

The real problem lies with my sister who I believe has BPD or NPD.
Since when did you start to think your sister has BPD or NPD? How was she when the two of you were growing up, was her behavior already problematic back then?
Half way there while she was driving with my BPD dad... yes another bad decision. They had a fight and she hit him and they were screaming at each other... so we had to pull over and me and my mom had to try and calm things down.
This sounds like quite a dangerous situation! Fortunately no one was (physically) harmed.
I do not feel too bad about it though as I think I needed better boundaries with her, I do feel guilty and sad that we cannot have a relationship but it's like a weight is lifted from my shoulders.
You do feel like you have good boundaries with your dad, what's the difference with your sister? Do you feel like it is more difficult to set and enforce boundaries with her than it is with your dad? Perhaps you can benefit from some resources we have here about boundaries:
Getting Our Values and Boundaries in OrderExamples of boundariesWelcome to bpdfamily