AIBU to feel hurt my cousin hid booking her long hoped for trip?

I agree. I've had this and it seemed like they either wanted to maintain a completely hard done by image with me, or, they really had just got used to me as their support and nothing else. It was puzzling hearing about all their work promotions and travels from other people, when all I got was doom and gloom.

I'd raise it casually as suggested above ("great news!") and see if she snaps out of it.

This

I don't get why you want to catch her out, why does it matter? She told you she wanted to book this trip, you gave her advice, she's booked the trip...surely you knew she was going to book the trip so what's she done wrong?

It also seems strange that you believe she's not been in contact as much because she's booked a trip, maybe it's something else?

You would you fall out with a close family member because they booked a holiday and didn’t mention it? That’s bonkers

you can just play it like on Friends.

don’t tell her you know and eventually she’ll know you know. (her mum will probably mention it to her)

and see if she asks why you didn’t tell her you knew.

This doesn't make sense. Why would she hide it if you are as close as you say? Is there more to it?

Have you given her money as you said she comes to you with financial problems? So perhaps she is worried if she tells you about the trip you may mention that she hasn’t paid you back but can afford this trip?

Maybe she thought you were wanting to join on the trip and she didn’t want you to.

maybe she went against some of your advice and thought you might take it badly.

she only booked it recently, why do you need to know?

I'd probably find it strange if someone messaged me to tell me specifically that they'd booked a holiday she probably doesn't realize you'd want to know. I wouldn't think to mention it unless it came up as part of a conversation in the moment. She's probably not avoiding you she may just be busy.

This is the most straightforward way to message.

Why are you making this into something more OP?

Do you think she feels a bit embarrassed that it’s not something special for you - you travel all the time? And just wants to savour the excitement because it’s really special for her?

My gut feel is that she doesn’t want you controlling it - eg “oh you are off to Malta - wow, I must tell you the best places to go”.

she is either finding that overbearing or has had a tough time and wants to be more independent? Or perhaps she just uses you when down which isn’t very nice if so!

Are you upset shes only interested in talking to you when she has a problem because that i would understand

I can understand why you're miffed she didn't include you in a decision that's a really big thing for her.

I can also see, though, why she didn't. This really big thing to her isn't that big of a thing to you. You travel a lot more than she does, and you've even done this exact trip before. You have advised her on it ... in her shoes, I might feel you'd squash the excitement a bit by giving too much advice and/or telling her "When I did this, I ..." making it about your memories rather than her anticipation.

I realise you may feel this is the last thing you'd do. All the same, you have very different parameters when it comes to holidays and it could be difficult for you to enter into her mindset when talking about her forthcoming adventure.

You can always just tell her her mum mentioned they were going, wish them a fantastic experience, and move along to a different subject.

Is she your cousin or a friend - you use both in your OP?

Does it have to be that significant though? You waiting to catch her out or test her by how long she takes to tell you sounds a bit odd….

I’d go with a quick message - ‘oh I bumped into your mum last night and she mentioned the trip to Timbuktu you’ve been planning - sounds fantastic!’

I also don't understand why you need to know about this trip. If she really is avoiding you, it's all rather odd.

If it's bothering you, call and say that you haven't seen much of her lately and have a chat/arrange to meet up. You can casually say that her Mum's mentioned the trip.

Why on earth would you be offended that your cousin hasn’t announced a holiday booking to you? Why would she need to?! It’s her holiday, not the birth of her firstborn child ffs.

This. Why does she have to tell you? Isn't she allowed any privacy, or to make her own choice when she will announce her holiday.

You don't have a right to know. If you are her friend, let her tell you in her own time.

Why do you need to know? Is it really that big a deal?

She is doing something for herself it is not about you

I can see why she hasn’t told you yet. She’s obviously hyping herself up. You might overvalue the relationship and smother it. Calm down.

Is it possible that, as you have done this trip twice before, she might think you will try to bombard her with advice and tips? That maybe you will "take over" in a way telling her what she must see, must go to, what you did when you were there.

Perhaps she will feel it is more special if she is not just following in your footsteps as it were.

In those circumstances I would probably just wait for her to tell me.
Then say “great” - and immediately change the subject…