even my husband cannot understand what I have been through, he seemed shocked that I wouldn't take the "opportunity" to talk to my SIL & nieces
Yes, it's near impossible for those not directly affected by a person with BPD to understand. Here you will find understanding... .as we all have direct experience in dealing with the illness.
I am not responsible for his delusions, behavior or how he treats his wife or kids
You are correct, and it's very healthy you are able to recognize that fact.

I could have made more of an effort & I didn't because I was angry... .so sick & tired of all the BS.
I feel like there us no true resolution even in NC.
Setting a boundary of NC is self protecting, and many of us here have found that a necessary step for our own mental, emotional and even physical health. Remember, HIS actions forced the necessity of the boundary.
it is hard to not feel like I could have done better for my nieces sakes
This seems like the crux of why you are here... .would that be correct? If so, what are you feeling on the subject? Are you thinking of dipping back into the relationship for the sake of your nieces? Or are you just venting? Either answer is perfectly fine, feel free to share.