I think you have done more than ANYBODY could reasonably expect, including your sister. Actually, especially your sister. There is one other person you are going to have to let off the hook, you.
I have a sister in law, She is HUGE. As I've explained to my wife over and over and over for 40+ years, she is ill and she is also a meen person. She had three kids. Katie died of a brain anurisnm at age 8. The really didn't help and Loa got worse. The next one Kevin died of, you got it a brain anurism at age 17. She went off the deep end. The last one, Brian was a bit of a sociopath. He'd just got out of prison and went to see his mom for Thanksgiving. Complained of a headache, and fell over dead with exactly the same damned anurism.
To this day she will not come to grips with the fact it HAD to be genetic and that nothing she did or could have done would have prevented it.
In the interum she gets angry at everybody in her immeadiate family and takes out her unhappyness on them. My wife would try and do things with her and would end up a wreck for weeks from just being around her.
After our daughter died of an astma attack Loa was vindictive and meen rather than supportive. I talked to the wife psychiatrist and gave her some insight because I knew my wife never would. Telling family secrets/tails and all that B.S.
When the wife went in the next time they talked, at length. The shrink advised the wife that NOTHING was going to make Loa happy and that because she was miserable she felt everybody else should be also. She recomended/insisted that my wife sever ALL contact with her sister in otrder to save her own life and sanity. Essentually she had to divorce herself from her sister.
We haven't had ANY contact with the sister since 2002 and I suspect we'll go through the rest of our life without contact. To do anything else is simply poisonous for my wife.
I know you feal bad about your nieces and nephews. Bottom line, there isn't much you can do and there is no reason to beat yourself up over it.
I agree the kids are probably sick/crazy. The sad part is that with most people suffering this that as soon as they are on thier meds they feal better. When they feal better they don't think they need the meds. It's a vicous circle.
I can relate a bit. When my daughter died I was right on the edge of suicide for about 3 years. To be honest the only reason I didn't was I KNEW it would kill my wife and son. So at least I was that rational. When I finally saw the shrink the first question he asked was exactly what could I have done to prevent her death? In a moment of clarity the answer was nothing. "So quit beating yourself up, 24-7-365". He did put me on anti depressants. It got rid of most of the suicidal ideology but it did make me "flat" no sorrow or grief but also no joy or laughter. After about 18 months I complained to the shrink about that aspect of the meds. His response? If you are that self aware, you don't need them anymore.
Take care of yourself. You can not solve the worlds problems and you can't solve your families problems, sadly they are beyond anything you can do and you've tried damned hard.
the best
Ross W Thomas