to feel hurt at being left out by family members?

Lol. Posters on MN are so desperate to blame OPs, they start concocting wild theories!

Really? I think it's quite thoughtless. OP's family sound quite close. And this was a family event as the kids attended too. It's something that should have occured to them...and it seems like it has in the case of one sibling, albeit too late.

Unless it was pure coincidence and they all just bumped into one another there...but that seems unlikely?

Can you say to your closest sibling that you were a bit hurt by it? Ask if it was maybe an impromptu thing (to give them an out) and say you were sorry to miss out and would like to be included in future.
I don’t think it will cause any fall out if you mention it to one sibling in a non-confrontational way. It could well have been a last minute plan that simply worked out that way, with no ill intentions (such as friends dropping out and extra tickets being available, but not enough for all of you)!
Sorry you’re feeling this way. I understand why it’s upsetting.

OP, I think some people just like to wilfully misunderstand. Your original post, and further responses are all quite clear. And I would have been hurt just like you, and I also wouldn’t bring it up with my family for fear of upsetting them, so I get exactly where you’re coming from.

I think you’ve hit the nail on the head as to why you weren’t invited.

Your family member wanted to go to Festival X. Probably because they enjoy the music and the vibe. They mentioned Festival X to other family members who were also thinking of going. They also like the music and the vibe. So they arranged together.

You, OP, have no interest in Festival X. You aren’t interested in the music or the vibe. You in fact did not go. Your only interest in Festival X is “spending time together”. That’s not what other people who went wanted. They wanted to wander around, see the bands, eat junk food, buy overpriced crap and not have to look after a festival newbie who’s only there because it’s a family event.

Don’t be sad about not being “invited”, it wasn’t that kind of trip. It wasn’t about spending time together as a family (the thing you enjoy), it was about going to Festival X (something everyone knows you’re not into.)

I don’t get invited to watch the football or go wild swimming or to Brazilian BBQ. I don’t like those things and people know it. They go with other people who like those things. Sometimes it’s not about spending time together as a family, it’s about doing the Thing, talking about the Thing, enjoying the Thing.

If you aren't prepared to say anything then there's nothing you can do and I would think your hurt and resentment will just fester

You never said a word when they said they were going. You weren't interested - you just didn't want to be left out.
You've also mentioned you don't drink as much as them - which tbh doesn't seem very relevant. How much do you mention this around them. Do you tsk or comment on their drinking every time? Are you a stickler for mealtimes, toilet hygiene etc.

This! All the other siblings were obviously invited- did any of them not think to say " Is OP coming?"

It is quite hurtful that the whole family went and didn’t think to invite you and your family. I’m sure they’d have all felt the same if it was their family left out. I’d be stepping back a bit from the family chat group this week as I’d be feeling very hurt. One of them should at least own it, apologise and explain why they didn’t invite you, as pretty sure they must have realised that one sibling wasn’t invited and the rest were.

You specifically said in your OP you've never shown interest in this kind of event

When DB mentioned going the others probably expressed a kind of "ooh maybe we'll tag along"

I doubt it was the case DB invited the others and left you out. Stop thinking of it as being "excluded".

And seriously, just say something.

"Looked like you had a good time at the festival. I didn't realise you were all going. I know it's not usually my thing but I'd like to have come with all of you. Let us know next time"

Yes. Though I'm not even sure it's particularly thoughtless. In that scenario, I wouldn't be thinking 'Oh, one sibling is left out' because certainly my siblings and I don't go around socialising as a 'set'. If the OP has never expressed any interest in attending a music festival, it simply wouldn't occur to me that she would want to go, just because all her siblings were going. To me that's quite odd, that something she has no inherent interest in would suddenly become appealing purely because of who else was attending.

Nothing wrong in saying next time I'd love to come x

But this is contradictory.

If you are close, then you are close enough for you to be able to say "Festival looks great, I'd have liked to go- if you organise something like that again, could you make sure you include me?"

If you're not close then this is a non-event.

Totally understand why you're hurt OP, I would be too. It was likely thoughtlessness on their part rather than deliberate exclusion, but you should mention it so it doesn't happen again.

Your upset is understandable.
They never mentioned a word in the days before they went which could only be deliberate.
Of course it would have been nice if they said, it looks like we are all now going, will ye join us.
Of course you are upset.
Pity that some posters have to be so unkind about your relationship with your family.
It sounds like it might be thoughtlessness on their part.
I think it would be wise to ask "is there any particular reason we weren't included in this gathering which we would have enjoyed"

Let them answer. You can ask without anger. You can respond with "I am hurt and disappointed to feel excluded and an afterthought, we would have really loved to have joined you if asked".

I think until you ask, you will not know and that will sour things.

Yanbu to feel hurt and left out when these are people you teams call several times a week.

I'd have felt really hurt my this too. It probably just kind of happened rather than any conspiracy to leave you out but that doesn't stop it hurting.

You were told one of them was going and presumably at that point you didn't say that you'd all like to go too? Perhaps your other siblings did express an interest in going and that's how this all ended up happening?

Just tell them that you saw the pictures and it looked fun and to let you know if anyone goes to anything similar/is going next year as you'd all love to join them.

If you thought you were all that close...ask why your family weren't invited, without being defensive...then judge their replies.

I think the problem is that you knew one sibling was going and at that point didn't say oh that sounds like fun, we like to come along too. You have only now decided that you would have liked to go along because everyone else was going which is more like fomo than actually wanting to go. I can see why they didn't ask you because you had already been told about the event and showed no interest in it.

What I would do is just say oh it looks like you all had a load of fun, we would love to come along next year. I expect they will be surprised that you have said that as you've given them no reason to think you would have wanted to go.

I think you need to talk to them. There’s a big difference between them all happening to decide to attend, and them excluding you, which implies collaboration.

Sounds to me like one family was going, a second and third decided to attend. None of them thought about the implication that you’d be the only one not attending. Which is a bit thoughtless, but it’s not purposefully leaving you out, which is how you’re expressing it.

Personally, I’d just say “event looked lots of fun, we’d have come if we knew everyone was going!”

I hear you @user092521 . You're upset because it feels like your family colluded behind your back so that they could go. There's a sense of betrayal and being deliberately excluded. I expect you feel if there was no intent to exclude you and your family why was there no mention of it in your usual family catch ups?
Your brother mentioned it because he always went. The other siblings didn't mention it which you would expect, if only to express their excitement at joining him. It feels off and it feels deliberate but as others have said you need to address it. It's already bothering you and the awkwardness already exists and will continue to linger. You've said that one sibling seems to have picked up on that already.
They are, more than likely already discussing it separately as they clearly did with the planning of the weekend. Just ask them when it was arranged and was there a reason you weren't included? Mention you have seen the pictures and are puzzled. If you don't address it now it will continue to bug you and make communication difficult. Good luck.

OP are you normally a people-pleaser? Please don’t be a doormat. You don’t have to be confrontational. But it’s important to speak up and express how you feel sometimes. This could be your first attempt to assert yourself, esp if it doesn’t come naturally to you. The world won’t end. Please consider speaking to one of them about it.

Really? I have been to stuff that I wouldn’t normally go to, as a chance to catch up with friends and spend some time together. It’s usually nice to be asked and included, as long as there is no pressure involved.