To not go to the family barbecue

I agree with pp, I wouldn’t go either.
And your sister is indeed a cow.

And as someone who was bullied by the ’cool girls’ and no one cared or helped me, I just want to say thank you, for believing your daughter and sticking up for her.
It’s going to be a massive thing for her, moving forwards and for her self-esteem.

Your sister should never have done that but I'd actually be very tempted to go and put on a show of support for your DD in front of them.

Her son won't have told her the full story, so I'd make her aware that it's not teenage squabbling; it just stopped short of the police being involved. Then let her decide how important they are. If your daughter doesn't want to go, that's the e end of it really.

‘Hi D-sis, these girls have called her names, taken and shared photos of her, pushed her, pulled her hair, kicked her….would you socialise with people who had treated you like that, honestly? I’m not trying to dictate who your DS is friends with, but I am going to respect DD’s completely understandable wish to have nothing to do with them. I’m not interested in them ‘realising she’s not a nerd and wanting to be friends with her’, she is worth a million of them and they don’t deserve her friendship. Hopefully see you and DS another time soon.’

WHAT?! My sister wouldn't even entertain my niece being friends with my daughters bullies and they're at the same school. If anything my niece has had to try VERY hard not to teach these girls a lesson! Your sister is VILE my sister wouldn't allow herself to be in the same space as my daughters bullies for any reason because she would, at best, tear strips off them!

I would be very clear to your sister that it's not "teenage girl squabbles" but your DD's life being made miserable by bullying. I'd say that she feels let down by her cousin who she thought she was close to , and that of course she is not going to attend an event where the bullies are present. And that you find it quite appalling that knowing the situation, Sis and her son have the invited the bullies to a family event, something where DD should be able to enjoy time in a safe space.

It's up to your DD. What does she want to do?

Maybe your sister doesn't realise the extent of the problem. It's very unlikely her son has told her.

If DD doesn't want to go then I would speak to my sister and say that unfortunately it's more than just teenage squabbles, it's been full-on bullying and the school has been involved. So it really isn't appropriate for DD to attend.

It's up to your sister whether she wants to encourage this friendship between her son and them.

Let's hope it rains!

I would not take DD to this - she doesn't want to go and has sound reasons. It's not just a teenage strop.

Absolutely wouldn't go. I'd take your daughter out somewhere instead. Your sister is being horrible, no way would i invite someone who bullied a family member.

I wouldn’t go and if it was my sister she would go down in my estimations encouraging her child to be friends with bullies.

You've got to be guided by your DD. Your DN being a teenager will not see it for what it is and I can understand your DS letting him invite who he sees as friends. The school should have stepped in sooner, it's got carried away. I wouldn't demonise these girls, kids make mistakes, putting teens into 'cool kids vs nerds' isn't helpful. Their behaviour was wrong and your DD a victim, so she gets to decide.

You are absolutely not being unreasonable. I’d say to your sister that these kids are bullies and you will never ever force your dd to spend time with bullies. How would she feel if her DS was physically bullied and you decided to host them and expect her ds to play nice and come along? It sounds like she’s massively minimising the whole thing saying it’s “squabbles”. You’re absolutely right to protect and support your dd in all of this.

I wouldn't go. Can you host a bbq yourself in a few weeks and let your dd invite a few friends? You can always invite your sister and nephew as well - that way you get to spend time with them on a nice environment

YANBU and your sister is a cow. Do something lovely with your DD.