AIBU not to be more cross with DD?

[shallichangemyname]

DSis and I have always had a fractious relationship. She is a stroppy sort (to put it mildly).
I have 4 DCs. She has 1 DD, who is same age as my DTs. DN has always been a very difficult child. DSis doesn't handle it well and none of us ever want to challenge her or even try to broach it. DN lies to get my DCs in trouble, has terrible tantrums (even at 13), singles out DS2 for nasty treatment (in the form of continual nasty comments - eg whenever he speaks she'll say "nobody's interested in anything you have to say") and always has to have her own way. She has terrible social skills which are not helped by DSis's approach to parenting. I encourage my DCs to make allowances and to be fair they are very good but they find her visits hard.
Unknown to me or my DCs DN has been having difficulties at school which DSis thinks is bullying and they have changed her school. Having g had no discussion at all I have no idea what form this bullying has taken and I also have no idea how much of it is true (there's a fine line between being bullied and just not being popular and having a few spats with your classmates).

St the weekend DN (who lives 4 hours away) joined my DD into a group Instagram chat between her and the alleged bullies. My DD knew nothing about it and doesn't know any of these people and had no idea why she was joined to the chat.
DN left the chat.
My DD asked what it was all about and when told rather meanly said "I don't like her much tbh".
First I hear of all this is last night when DSis sends me a screenshot then phones me to tell me about the bullying, how DN had wanted support, how upset she was by the comment (which someone screenshot and sent to her), how my DD should have backed her up etc. I asked why my DD had been joined to a chat where she knew none of the others or what the background was and this didn't seem right to me. DSis started ranting so I hung up. I sent her a nice message to say I couldn't speak to her if she was going to shout and that I still didn't understand why DD had been dragged into it. She messaged me back to say DD was a "little cow" and took after me because I "don't give a shit".
DF returned from an evening at the fireworks. She was feeling a bit guilty and had already messaged DN to apologise but was just totally bewildered by being joined to the chat, with no forewarning.
Sorry for the rant. AIBU in not being really cross with DD? She understands she was wrong but I think DN was wrong to involve her. What do I do re my DSis?

DSis doesn't handle it well and none of us ever want to challenge her or even try to broach it. DN lies to get my DCs in trouble, has terrible tantrums (even at 13), singles out DS2 for nasty treatment (in the form of continual nasty comments - eg whenever he speaks she'll say "nobody's interested in anything you have to say") and always has to have her own way. She has terrible social skills which are not helped by DSis's approach to parenting. I encourage my DCs to make allowances and to be fair they are very good but they find her visits hard
You're a doormat and you've taught your children to be doormats. I feel so sorry for your children that you've stood by and allowed them to be bullied by their cousins and aunt because "they're family".

Your parents failed to stop this in its tracks when you and sister were children and now the cycle of poison and toxicity has continued down to a new generation. Stop making excuses for Niece. It's irrelevant that she's an only child. None of the only children I know behave like this.

But do you not think it's natural to be more tolerant of your family?
No. I have cut out family members whose behaviour is unacceptable. I won't tolerate it in it strangers, friends or family. Sharing some DNA does not give anyone the right to treat you badly.

Bullying/bad/unacceptable behaviour is unacceptable regardless of who does it. And I would never, ever, in a billion years, accept it if it was aimed at my children. I'm their protector. If I (and DH) don't stand up for them, who will?

Niece and Sister's reactions to the instagram post is a typical response of people who can dish it out but not take it. Do not apologise for it any more. Have they ever apologised for the nasty and cruel things they've said or done? No. So follow their lead and move on. Be glad they've blocked you. Don't back down and allow them back in. Move on and relish a life free of toxicity.

Maybe it wasn't the best thing to say to a group of strangers, but it sounds to me like your daughter has just been honest when asked a question. Sometimes the truth hurts. Your niece is partly to blame for the situation. She tried to engineer a situation to her advantage, but by not telling your dd her plans it backfired on her.
Your niece doesn't sound like a nice person to be around if she speaks to people as you have described. It also sounds as if her mother can see no wrong in her, so everyone else must be a bully.
I can see you did what you did to avoid upset, but maybe the children would have got along better if your niece /her mum had actually been told how upsetting her behaviour was for everyone else do it could be addressed earlier.
I would stay quiet for a while and see what happens.