To be upset by this reason for not coming to my wedding?

I wouldn't waste some precious annual leave on anyone's wedding abroad. It would mean me paying to go somewhere I hadn't chosen and would probably mean I couldn't have a family break.
The facts are nobody really cares about your wedding in the way that you do.
It is selfish to expect people to use leave, pay for flights and accommodation and leave their family at home just so you can get married abroad.

You are asking her to use money and annual leave for a holiday / wedding in Italy that her partner is not invited to.

I hate using my holiday budget for 2 or 3 day trips as it is such bad value . If I pay for flights I want at least a weeks holiday to make it worth my budget. Italy is expensive and I don’t want my holiday choices made by people getting married.

She will no doubt be at your party.

But people generally see these as the runners up prize, not the main wedding.

You can’t have your (wedding) cake and eat it.

You can’t have a destination wedding and not invite partners. That’s a very self centred thing to do. Your mum/sister just thinks it’s a shit invitation and is using the dog as an excuse because she’s too polite to tell you how rude you are for not inviting anyone’s partners.

When having an abroad wedding you have to accept the fact that any member of the people you want to be there could so no for any reasons at all.

She may have big anxiety issues with leaving the dog/flying. She could be currently trying for a baby or pregnant. She could have major money issues. She could be in the middle of splitting from her partner. You never know.

But with an abroad wedding you cannot hold a grudge against people who don’t want to go. It’s the risk you take.

Why can’t you have a wedding at home and honeymoon abroad??? That’s the normal order of events. Not forcing people to spend time and money to haul around so that your ceremony has a foreign backdrop.

When I had dogs it was £100 a night for my live-in sitter. Nobody’s wedding is worth that to me, plus all the other expenses.

This ⬆️, absolutely.

CBA reading all updates but if you get married abroad (especially if it's just because you like It there, not because you're each from different places and have to pick somewhere) you really have to go into it with absolutely zero expectations from family and friends. It's a big ask compared to a domestic wedding and you can't make any assumptions. She's said she doesn't want to go, that's your response from her. If that changes your enthusiasm about getting married abroad, that's that.

What about a honeymoon there instead?

you cannot possibly know why people would decline @Owlicecream88

It is absolutely not about how much they love you.

A destination wedding is often used as a holiday for guests due to the expense. I think it’s a huge, and inconsiderate, ask for people to attend without partners.

Everyone doesn't "hate weddings", @meganorks I've already said upthread that I love them!
I think people are making valid points for the OP to consider, I don't think hate is being displayed.

OP, just get married and have your celebration in the UK,and then go on honey moon to Italy.

Our wedding was 5 minutes down the road from my late MIL. She was elderly to be fair but didn’t come, used to go to the garden centre on a Friday though. 🙄

Can you not just get married here and have a Honeymoon in Italy? People do this for a reason, it works.

If she has a dog and her partner works all day then what do you suggest she does? That her partner has to take annual leave to hang around with the dog? That they pay for a dog sitter/walker on top of the cost of flying to Italy? That they leave the dog barking non-stop and possibly wrecking the house and pissing on the floor the whole time because it’s not used to being alone all day?

A dog is a responsibility. People can’t drop their responsibilities because you happen to want a destination wedding.

This is not really a "normal" situation. They're being asked to fly abroad without partners or kids to attend a wedding only. The celebration is happening in the UK.

I wouldn't go in this instance,I'd say, go and get married, we'll celebrate when you're home.

Your are viewing this solely as the bride for whom the wedding is everything.
It isn't like that for everyone else, even your parents.

Destination weddings are ultimately selfish & you can't expect everyone to be as on board with the idea as you are.

You say it's "just flights" but it isn't. It's using annual leave, it's buying an outfit, it's buying a gift and no doubt spending money while in Italy.
Weddings are expensive for guests no matter where you have them but planning one abroad adds an extra level of expense & hassle. Plus saying this person cannot bring their partner and expecting the partner to happily syay home & care for the dog smacks hugely of selfish entitlement.

As other have said, do it the other way round. Have the wedding here & a romantic honeymoon abroad.

You can't invite adult siblings and not invite their partners. That's ridiculous and I would rather be at home with my dog and partner too!

I wonder if you are the first to get married in your family, or are others married already?

What kind of commitment did they ask of you when they got married? Local venue so you could go home, or further away and needing a hotel stay? Or foreign wedding needing a larger financial commitment and taming leave off work?

MN isn't really the right place for this kind of question as everyone seems to hate weddings, especially destination ones.

I absolutely understand why you are hurt by this. I suspect that the dog is just a handy excuse though as I don't think it would be a straight no for that alone with 2 years notice. And like you say, it's the phasing of it. If she had said something like 'I'd love to come, but I don't know if it will be possible because of the dog' or similar that's very different. Is she someone who travels abroad at all?

Incidentally, I had something similar with my dad. It wasn't a destination wedding. The venue was close to where I live just not that close for my family (c 2hr drive). My dad was pretty old and I knew the full day might be a bit much for him so I spoke to him about it. The venue had some rooms so I said I'd love for him to come but was happy for him to do as much or as little as he could manage. And he could have his own room to retreat to if and when he needed to. And my sister would take him. It was over a year and a half before the date. He just grumpily muttered that he didn't know about that. I didn't expect a definite yes or no. But it would have been nice for him to say he wanted to be there. But there was no acknowledgement of it being a special day he just was already thinking of what a massive inconvenience it was. I was very hurt.

I can see that in your head this is making it fair to all of your guests, but it doesn't sound like much fun to go to a wedding without your partner.

If the roles were reversed how would this make you feel?

There’s no way I’d attend a destination wedding without my partner. It would eat into my leave allowance and the family budget. I might make up an excuse to get out of it rather than tell you the truth though.

Destination weddings are annoying. I could not go to one no matter how close I am to them, unless they paid all expenses for me, including my dog arrangements. The whole thing is a hassle to be honest. It’s the risk you take when you go abroad to get married.

Honestly, planning a wedding really shows who cares and who doesn't. I had several family and friends not come for shitty reasons.

I love weddings, even abroad weddings! But, I would not go to a wedding abroad where my partner wasn’t invited and he would have to take annual leave to look after pets. I’d probably use the pet excuse to you so I didn’t have to say, ‘I’m not coming because my partner isn’t invited’. I’d go to a wedding a few hours away on my own, but it’s a big ask to take leave, pay for flights and go away without my partner.

You asked, she said she can’t go, I don’t think you need to be upset about it, it’s not a personal slight and they can both come to the celebration at home.

The Italy wedding seems to be just for you two, why not make it your honeymoon rather than ask people to go away solo without their partners where they’ll be a bit sad - it’s not the atmosphere you’d want at your wedding surely?

It sounds like this family member is your sister? (As you mentioned only parents/grand parents and siblings are invited)
if so, I understand her not wanting to go, you’ve left out your future brother in law.

the only person this wedding will be easier for is you, so you’re going to have to expect people to decline the invite.

maybe just go with your parents and leave it at that for the wedding and have a party at home.