Sisters 21st or partners 40th?

Really stupid to book the trip on the weekend of her birthday when there would be a good chance that she’d want to celebrate with family.

Rearrange the trip.

Go to sister’s party on one day of the weekend and then take your partner out for a special lunch/dinner on the other day (with the trip the following weekend).

My opinion is that there's no need to take your sister out for her 21st and spoil her like you did for her 18th, because she's clearly already spoilt enough. She's being utterly unreasonable. You can celebrate her birthday on another weekend, as you already have plans, but the fact that she's thrown a strop over this would seriously make me not want to bother. She's asking like a toddler, not an adult—except most toddlers I've met wouldn't be as bratty as she's being about this.

A 21st isn’t that big of a deal to me. She’s already classed as an adult. At that age I’d never want to celebrate it with family anyway. I’d have been out with friends. She’s being precious.

Sorry your sister has booked and arranged a party in a week or she's had it booked for a while but only decided to invite you a week ago... hhhhmmmm

Your sister needs to grow up

Your sister needs to grow up. She's 21, not 12.

This.

She’s being a brat about it so on principle should not be rewarded for that behaviour.

A holiday abroad (of any length) booked months ago takes precedence over a party announced a week ago. Add on that he's your long term partner and that trumps a younger sister's birthday party, IMO. Don't incur costs to placate what sounds like an immature girl. Just tell her you'll have a small dinner or similar on a different date, and make sure she has any card or gift before you go. Have a lovely time in Italy.

Well, a partner is always going to be more important than a sibling. So there's that.
Plus, I think the Italy trip was already booked, so a prior commitment always takes priority.
The sister is being U..... the OP can do something with her another day, so I'm not sure why the sister thinks it's such a big deal.

It was 16 for females and 21 was the end of warship for males

But did you check it with your sister re her 21st birthday plans before booking? That’s what I think I would find hurtful. You must have known the date?

Don't change it; your sister's being a complete tit and needs to grow up. She's 21, not 5.

Firstly, she hasn't 'deliberately missed' her sister's party. She booked a trip to Italy, for her partner's birthday. Who said the sister's party was arranged first?

Secondly, why would her sister's 21st (given that she already did a big celebration for her 18th) be more important than her partner's 40th? The sister is having a party, loads of people will be there, one person not being present won't make much difference. The sister is going to be partying with her friends most of the evening, I'd imagine. But the OP's partner isn't having a party, so why should he forgo his birthday trip to Italy to celebrate someone else's birthday instead? I'd put my partner first in this situation every time.

The sister sounds like a spoilt brat to me.

Are you the sister?

Whilst she wants you there she wont spend much time with you and will probably be with her friends for the majority. She wants your presence rather than to share the experience with you.
Personally, at my age, I’d deliberately book a weekend away to get out of going to a 21st birthday party.

The reason 21 was considered "adult" was because that was considered the age when a man could weft a sword. God help us if the government do lower the age of consent to 16.

It's really not. There is nothing you can do at age 21 in the UK that you cant do at age 18 so why would 21 be big deal? you're already a legal adult then!

May be true for your part of the UK and your bubble. But I am from a very traditional British family and a 21st can be a huge thing - formal black tie event with sit down meal for 100, paid for by parents, speeches by godparents, etc. Not dissimilar in scale to a wedding. Influenced by Edwardian British traditions rather than modern American ones. My kids weren't interested and we didn't have the budget anyway but in posh British circles they're definitely still a thing.

She says in the OP that the Italy trip is just the two of them

Because one action is different from a pattern of behaviour. Not everything is “abusive” and not everyone is a narcissist etc etc. Sometimes we can all just behave a bit shit. No need for the buzz words

These responses are absolutely mad!!!!!!

When my friends and I all turned 21 we were at university. My Mum & Dad sent me flowers and a piece of jewellery. I saw them when I went back home for the Easter holidays. I think my sister probably sent me a card?

21st is not a big deal, it just sounds like a nice number. It used to be legally significant in the past and it continues to be in the US. But we are not in the past or the US.

Don’t change the trip your sister is being precious

Presumably you have not lived with your sister for years as you have children and have been together for 11 years. I would apologise for the oversight but stick with Italy.

I assume she would spend most of the party with her friends anyway.

historically 21 was coming of age (become and adult)and the big birthday to celebrate - then the coming of age was lowered to 18 years old and so 18 became the big birthday.

Clearly the Ops sister wishes to celebrate twice, which is her prerogative - but other might only want to celebrate once

to say the change of coming of age happened in 1970 in England

You’ve known for 11yrs your hubby and sister have very close birthdays. You’ve also known his 40th and her 21st were big birthdays looming and were going to be the same weekend! You’ve also known booked a break away that he’s not bothered about but your sister is. I think YABU as you knew how old they both were and when their birthdays are so you could have attended her party and took hubby away the following week…IMO you’ve been thoughtless.