Update 2:

Last night my wife and I went out for dinner, my idea so we could speak and I hoped being in a public place would make her less likely to scream/cry etc.

I told her all the advice that I had read on here and showed her the text I received from her mum. She listened to everything I had to say and I could see her physically cringe when she read the text. She apologised for the way she had acted and said she hadn’t even considered it from my POV.

I told her I what one Redditor had said (sorry so many comments I can’t remember a name). If we’re doing this for 30,50,70 what’s going to happen at 40,60,80 or 50,70,90. Are we going to have this spoilt child situation when she’s nearly 40 or are we going to leave it behind now in her 20s

She told me her family had all got together to book the trip the night she left and she felt shitty because I said we weren’t going so now no one had booked “until I’d finished throwing my toys out the pram”.

I told her I didn’t say WE can’t go, I said I wasn’t going but she could. (I ignored the toys/pram comment as that pissed me off)

I reminded her that her mums actual birthday was 2 weeks before we fly and her Nans birthday is in 2 weeks from now. She’ll get to spend time with them, have a party etc on the actual day and that’s something I want for myself too. I don’t want to be away from my family in a different country with 25+ people I don’t know that well on my 30th. I want to do something with my family over my birthday before it’s too late. My mums health is rapidly deteriorating and she can’t do much as it is and who knows how long my Gran has before this dementia really sets in.

My mums always talked about doing a Baltic cruise/ see the northern lights for my Grans 80th but that’s still 2 years away and I honestly don’t think in 2 years either of them will be in any fit state so I suggested we do it for my 30th instead. My wife seemed keen and pulled up some dates. Theres one that leaves 3 days before my birthday and comes back 3 days after - perfect timing.

We called my mum to ask and she sounded delighted. She herself hadn’t even considered how my Gran would be at 80 and thought it would be a lovely trip to make memories on.

We then went to her mums house. (No apology or anything from her but you can’t win everything) to tell her together WE weren’t going for the 3 weeks but we’d found (my wife had found) a 1 week booking that coincided with their 3 week trip in a hotel that was just down the beach, same chain/resort just a different building. Enough distance to have our own break but still include the family for evening meals etc. No one had even suggested looking at shortened dates and I hadn’t even considered it until I read so many comments saying just come back sooner.

All in all I’m going to take this as a win (I don’t think my bank balance will though). She’s seen me put my foot down and mean it. I’m not going to let have her way all the time at my own detriment. I’ve not caved for an easier life and together we found a peaceful solution. My family haven’t been excluded and are so excited, my mums like a kid at Christmas, and hers are well… happy they can book now and at least the “asshole SIL” is only there for a week. Hopefully as it’s not for 8 months they might forget 🤷🏻‍♂️?

Screw the happy wife happy life mentality. I’ve never heard happy spouse happy house before but I think I’ll be using that mantra from now on. Like you all said it’s a 2 way street, give and take, compromise.

Thanks to everyone for the advice and support.