AIBU to feel hurt my SIL ghosted my hen organisers?

Oh absolutely, same for me. Obviously as and when she proves that she would be more reliable with plans, or grown up about cancelling or saying no to arrangements, then things will change.

Is she an awkward/shy person generally?

This is the kind of thing I would have struggled with at her age. I don't think I would have ghosted, but probably just told you directly or via your partner

She probably said yes with good intentions, then the planning and costs started and she felt very overwhelmed about it and didn’t know how to back out.
shes 24, give her a break. A 24 and 34 yo should have different ideas about money and you could be a bit more understanding.

She won’t be childish forever though. This sounds like you would write her off, but people tend to grow up a lot in their late twenties (I know I did, and I cringe over some earlier mistakes I made). I’m glad my family all still bother with me!

I’d be a bit worried tbh. She sounds like she might have some issues, at the very least around confidence and speaking up for herself.
You know her best OP. Is she the type of person who tends to be deliberately rude and uncaring? If not, there’s something else going on.

Sure, but, as so often on these threads, the only behaviour the OP can control is her own. The category of ‘What X should have done’ always excites endless wrath on here, when it makes far more sense to recognise that this is outside your grasp, and that you should concentrate on your response/stipulations to stop it happening again, if that’s likely.

I’ve learned from younger people no response from the beginning is a response. Non participation in the chat is a rejection of the invitation. It’s not rude to her generation (although some 50 year old mum will be along to say her 17 year old would never do that) and you have to learn the new mode of communication

That's just bonkers lol, and I don't care if I am "that 50 year old mum turning up to say my kid would never do that" 😁
It's just excusing away bad manners.
"Oh, it's just the way kids are nowadays, bless them."
If it's "not rude to her generation' then people are fine with young adults (and then by the passing of time) just adults in general ignoring you instead of declining. Because "that's ok, it's just what everyone does"
Teaching manners just out the window it seems.
Depressing.

Maybe but the solution is not avoidance. She always had the option to talk to the OP.

She is 24 years old not 14 - how does she manage feedback on her Masters’ dissertation if she can’t excuse herself from a hen do via Whatsapp / text?

I would cut her slack and just consider it immaturity. She didn't know how to say no to people 10 years older than her.

I’ve learned from younger people no response from the beginning is a response. Non participation in the chat is a rejection of the invitation.

That's going to go down like a sack of shit in the workplace so "younger people" who want to make anything of their lives need to snap out it. I'm aware it's a thing and have experienced lots of our trainees ignoring client communications or partner instructions if they don't feel "comfortable" with what they have been asked. They get pulled up and are told if you need guidance/clarification/support, that's fine, just ask, but ignoring something you don't fancy is totally unacceptable and risks damage to the business. Some of them get it and end up doing well. Some just don't engage. Guess which ones are never offered a permanent contract?

Really bad behaviour by your DP.

It was rude but I suspect that she muted thread and did not see the queries. I find that generation are not good communicators. You DP should have contacted her directly rather than the family thread. Looks like her mother got in first to reply so I don't think you can fully blame her on that. Hopefully she will be your sister in law for life so I would just move on and forget about it. I hate to say it but your Hen is important to you but for others it is just another thing to fit into their diaries.

I had the opposite experience, I knew it was unlikely my 2 future sister in laws would be able to attend my hen, so I didn’t ask my bridesmaid to include them in the invite list, as it also would have created unnecessary chasing for her for people that wouldn’t have come anyway.

Both had a go at me after saying how upset they were at not being asked, that they wouldn’t have been able to come but ‘it was the thought that counted’. I imagine if you’d left her out altogether she would have moaned too.

I think she should have said no from the outset.

I wonder if she felt she had to accept & was hoping to bow out later?

That said, she was ignoring messages so perhaps the hint could just have been taken?

I wouldn't do anything with just her & MIL.

Does your fiance not get on with her?

Yes, tbf this is also the family culture sometimes.

My MIL likes to coordinate events and allocates tasks/gifts etc for people to buy at events.

She handled it badly but it's hardly worth any drama. I've got a feeling a good number of people on this thread would go no contact with her forever over this.

If she can't even be bothered to send a single message declining (after initially saying she's live to be included), then I don't think I could really be bothered to organise a separate dinner or similar so she could feel included.

If radio silence in the group chat was meant to be taken as a decline, she shouldn't have said she'd love to in the first place. Too much of a mixed message. And I imagine if the organisers had taken the silence as a decline, closer to the time either you SIL or MIL would come storming in about how dare you leave her out.

I'd give her the benefit of the doubt. She might have something going on in her life that you don't know about. It sounds like maybe she's not coping well with something. Calling her out on the family chat wouldn't have helped.

My SiL still replies on behalf of her children when I send out individual invitations. Her children are 48 &50 and have adult children of their own. OP, she was rude but move on as it’ll end up becoming a bigger thing than it is, especially with MiL!

Sounds like she was more embarrassed to say anything. However she could have come up with something polite to let them know that she can't make it, or message you if she's uncomfortable that she doesn't know them well enough. She does sound a bit immature for her age, but I think this is a reflection on her and I wouldn't let it affect your relationship.

How far in the future is this weekend away? She may not have wanted to commit because she doesn't know what she is doing weeks or months from now. Since she is a decade younger and possibly don't know anyone else who is going she might not have been interested in the details of the planning so probably muted the chat. She is doing a masters and living a student life, hen do planning is probably very far down on her list of priorities. Sure she could have said earlier, but I don't think it was a huge crime not to pay attention to the chat.

Had she originally agreed in the group chat to participate then radio silence or had she never replied to any messages at all? If the former then it’s a bit rude but not the end of the world. A lot of pearl clutching in the comments about being rude . If it’s the latter I’ve learned from younger people no response from the beginning is a response. Non participation in the chat is a rejection of the invitation. It’s not rude to her generation (although some 50 year old mum will be along to say her 17 year old would never do that) and you have to learn the new mode of communication.

The fact this seems to be becoming a drama is ridiculous. It’s important to you OP but it’s not the same level of important to anyone else. The fact you think this will impact your future relationship is showing your immaturity too. It’s a hen do not the royal wedding and not the end of the world.

Very rude and childish of her, yes. I wouldn’t hold a grudge over it, but just go forward remembering she’s flaky, childish, and don’t put too much effort into arranging anything with/for her.

It is rude but I've come across an astonishing amount of people who do this. Their logic seems to be:

I can't do that anymore for x reason but I would feel rude/awkward telling them and I don't want them to be annoyed/disappointed.
So I'll just say nothing and hope they forget and leave me alone.

Completely oblivious to the fact that NOT saying anything at all makes things seem more rude and awkward. If the reason is one they are embarrassed about, it just draws more attention to it.

I know people still behaving like this late in life. My mother is one of them. If she gets called out on it she is suddenly the victim and the other person is "being unkind" to her.

I agree.