I'm going to very respectfully say that I don't think this was posted in the right place. I can't find anything humorous about anything that was written here. 
Because if I don't find a way to laugh it off, I will cry. And I've cried enough. As another poster wisely advised, it's time to move on. Maybe this is my way of doing just that - by finding what humor I can in the absurdity of it all. Because, to me, it is. Absurd. And dripping in irony. Because for most of my life I cared too much about what they think - and it ran my life.
Now it doesn't. This is me liberating myself from them.
And I'm sick of feeling bad about my family. They have been a source of great pain for me (in more profound ways than I care to mention) that it feels good in finally calling them out on some of the more ridiculous behaviors, even in an anonymous forum.
This is (my) way of finding a way to heal. Thank you, eHell. For me, this does belongs in the Humor section, because now I can finally laugh about it...and other stuff...and let it go. 
You said that you weren't there, I'm not sure why this upsets you to the extent you described above.
1. Because I love my nieces and the part(s) that affected them upset me out my love and concern for their innocent well-being.
2. Because I tried to prevent it. Alas, I failed, yet again, to do anything to help someone I care about because of past history of dysfunction, toxicity, and mis-trust in my immediate family.
3. Because I heard about it, from both sides, over and over again until the details were burned in my memory. And then I realized (recently) that
I didn't have to. And then, after that epiphany, I realized there were a lot of things with regard to family interaction/dysfunction that I didn't have to do - such as participating in any dysfunction - even as an after-the-fact witness.
4. Because of the realization, lately borne, that I added to my own misery by allowing myself to be subjected to any of this, thus participating in
victimizing myself - and allowing my time to be wasted on so much nonsense.
5. Because that's what family drama is. A waste of precious time and resources that could be spent more productively - and
happily.
6. Because it is not simply this one incident to which I refer when I say letting it go - I'm talking about the entire body of dysfunction with my siblings. This is
the one iincident (throughout a lifetime of such) that I wish to share. The rest, believe it or not, is too personal.
7. I'm laughing at myself for taking the whole rel
ationship with my siblings way too seriously (and seeking their approval, which would never come) for too long, when there is nothing I can do to change it.
8. Finally reaching acceptance that they are the way they are, and I am the way I am. And let it be what it is.
