To ask that my nephews don’t come

All the people who are just saying that the sister shouldn’t go…. Are you missing the fact OP is taking her THREE children all UNDER 5? No meal out with kids that age is particularly enjoyable, how is OP and her niece (that she’s oh so close to in age) going to have a good old girly conversation while OP has to manage three under 5s by herself. On what planet is that a nice 18th celebration.

I think it’s important to talk about it kindly (including with the non invitees) beforehand and depending on who they are, they could feel hurt/ left out or sidelined. It would still be ok to make the decision to do something they don’t want to be left out of and make it up to them by saying sorry and asking what they’d like to do another time. It’s your daughter’s birthday party. If they don’t really care then just give them a hug and have a nice party knowing you’ve been open and honest.

Just agree and say as this is a birthday gift it can be the boys birthday gift too, no problem if that's what they would like. They won't want to come to a girls tea party so they will decline.
Otherwise just ask her to a sleepover as another poster suggested, if that might be allowed.
Your sister is being rather odd.

I would say that the niece can decide to go on her own bat, and your sister can either like it or lump it.

How do people imagine this approach will play out? This sort of attitude leads to bad feeling, which doesn't benefit the niece at all.

It's a weird hill for your sister to pick especially seeing your updates OP that your niece is 18!!!

Your sister is being ridiculous if she compares a 6 month old baby to 2 teenage boys!!!

Keep it to the original plan your sister can either have a hissy fit or maybe realise spending some quality time with her DD is a wiser option!

My teenage boys love an afternoon tea. Why wouldn’t they?

Initially, I imagined the niece and the boys to be under ten and was thinking it doesn’t sound so good.

But no! With the 18 year old niece, can’t see any issues. You sound like you are a lovely involved aunt. Nice - I wish my siblings and siblings-in-law could take note. I would say that the niece can decide to go on her own bat, and your sister can either like it or lump it.

It isn't a school party!
The OP has excluded her own nephews without just checking with their mum/her sister before going forth.

This is a sibling argument (between OP and her sister) that was completely avoidable. Pretty shit for the niece and the nephews, especially because the OP is keen to keep the argument going.

Why don’t you let your sister have her meltdown and not come? Just take your mum, your niece and your children? At 18, surely your niece can attend afternoon tea with her aunt if she wants?

Did I read this correctly? You are 18 with 3 children so your sister is 32 with 3 teenagers? I think I missed something - the thread is long! Hope you get this sorted.

Surely if it was a school party the mum wouldn’t expect all the her DC to go. It’s absolutely fine to invite some family members to certain events.

Call it off and just invite DN over for a meal on her own.
Go out to meet up with others quite separately - not invoving your sister. Going out for a meal at a hotel at a time that doesn't suit the teen boys (sport practice night) could also work.

I often do girls day with niece but I then wouldn’t take my boys. So while I disagree with your sisters attitude towards the treat for your niece (and I get his only 6 months) but could you not have left your son at home so it was a girlie day

You judge your sister's parenting a lot.

Your sister is being odd. Id tell her its girls (and your baby boy) only or its cancelled.

I don’t think it’s very nice to exclude children from things and your sister has made it clear that is how she feels also, bit weird you didn’t discuss with your sister prior to inviting.

Nice idea poorly executed.

Edited to add - I have read through your posts and tbh you sound quite unpleasant about your sister and not supportive to her, you’ve been negative about her parenting and seem dismissive of her as a mother.

OP, your sister is so silly, shooting herself in the foot.
Let her be difficult.
Your niece is 18 now and you can have a relationship with her separately to her mother.
Your sister is a complete idiot on insisting her teen sons are included.
Yanbu.

Not good behaviour to 'dictate dynamics' between a parent and their children.

Keep it girls only.
Tell your sister that she is right and the girl day should not include your tiny son.

Arrange his Dad to have a fun outing with him, or they could go and visit the nephews and their Dad with pizza.

The son is a baby!!!

You'd think your sister would be really touched that you were planning something so lovely for your niece. Think I'd just say to your sister that your niece is keen to have a girls only treat so you can either go ahead with that plan or you can cancel it altogether. I wouldn't back down and include the nephews as it's ridiculous! It doesn't sound like they would even want to come anyway and it's not what your neice wanted either. Your sister is being really selfish!

should I include my nephews or stand my ground
It was an error to plan it without talking it through, 'standing your ground' would actually be doubling down.

I think it'd be healthier to just walk back the arrangements, it really doesn't benefit your niece to be caught up in whatever argument you're having with your sister.