Feeling left out

Likely because your parents were already there.

You seem to be looking for offense when likely none was meant

I doubt they’re trying to be hurtful but are replicating the family dynamics from how they live in the UK. Your sister is obviously close to them and they enjoy spending lots of time with each other, including the grandchildren. You normally live hours away and see them rarely and then when on maternity leave, chose to go to another country where I’m assuming they don’t visit regularly and you don’t return weekly or anything to see them.

In their mind, I’m sure they see you as being very independent and liking time with just your DH and kids. Rightly or wrongly, this will also impact how close you are and how normal it feels to spend days on end together. They perhaps feel like they’ve given you two weeks and now will do the same with your sister. In their shoes, I wouldn’t expect you’d be bothered since you don’t normally see lots of them. They no doubt want to sightsee on their limited time in Spain and perhaps felt you would have already done all this with living there, so it’s not the same. There’s also potential age differences between your children and your sisters which could play a part. Depending on their age and energy levels, maybe they feel like they need a rest after 2 weeks with a very young baby and if your sister’s children are older, this does change things and may feel more relaxing.

I guess the thing is that if you want more family time, it’s not just on them to provide it whilst on holiday. You could visit them more in the UK or have spent your maternity leave closer to them.

Why don't you suggest a plan for you all especially for your birthday. You know the area, perhaps they find it odd you aren't being more forthcoming and are expecting them to plan. Why not offer a couple of trip ideas or a meal at your home together. Communication can be tricky. If I came to your area I would want you to suggest something for a few days and have a few days separately.

And happy birthday for tomorrow. All the best people are Leos.

Why though? OP and parents had two weeks together without sister. Why should sister not get the same vacation experience?

Oh that changes things, I thought the way you'd put your pp just you and baby had come across.
Think @NerrSnerr is right, they are here for 2 weeks of hols, while you're living your 'normal' life?
So unless you/dh'd said 'planning this for my bday' they prob think you have plans and are in holiday mode as is only 2 weeks for them?

Are they assuming that you’re celebrating your birthday with your husband and children?

maybe I am being over sensitive and over reacting

No, you've just realised something that your brain had only touched on before. Before there was distance, now it's obvious.

Don't wait around for birthday stuff. Sort something out as if they're not there. You know the score with your parents now.

Look on the bright side, your sister can do all of the grunt work when they slow down and need help

If they don’t suggest any plans for your birthday and then ignore the lot of them.

Don’t message them again at all.

If you arranged something for your birthday, would they come? Birthday are important for us, yes even adults, so I would be sad if they missed this.

You can’t change families and their dynamics unless there is will on every side to change. I am sorry x

Happy birthday OP!

Would you be up for doing the kinds of things they are doing? A day at the beach, with a newborn etc?

Yes. I don’t think it’s deliberate but as DGM knows her other DGCs well and their routines etc she finds it easier with them-I’m presuming they were the first born DGCs. I always try to treat all DGCs equally but sadly some don’t.

So, DM and DSis see themselves as a family and you are an added extra sometimes. That’s very painful.

There are loads of examples of this on Mumsnet and you can run your emotions ragged chasing them desperately. Don’t try and understand why - good people never will. Just accept the fact that it is what it is.

But, now you know and its time for you to examine your own feelings and make a decision as to how you want to live in the future. Do you continue to chase for little reward or do you make your life in another direction?

I agree that your parents should include you and your family in all the outgoings. You can all go out as a big family, especially since you don't get that chance often in the UK. What's wrong with wanting to go out with your sister as well?

Is there a back story with your sister? Can you talk to your sister about it?

I can see why you’re upset. I expect your sister booked somewhere nearby because your thoughtless parents planned a stay with you then a stay with her and asked that she could book somewhere nearby you because they didn’t want to ‘venture’ too far on a coach or train in a foreign country (even though the Spanish bus and rail system is far more efficient than that of the uk!). So in their minds it was a done deal - a fortnight with you then a fortnight with her, not realising in that oblivious way that so many people of a certain generation seem to have on here, that you were also hoping to see them - and your sister - this week.
What is particularly unreasonable is that you’ve pointed this out and been effectively silenced by them by being told to grow up. Decent people wouldn’t do this; they’d say something like, “Oh shit, we’re sorry - we hadn’t quite realised you’d want to see us this week too because we were so focused on seeing you last week. Of course you just join in and see your sister too.”

I’m not surprised you feel sidelined op. Your family’s thoughtlessness is one thing, but to have your feelings brushed aside so rudely is quite another.

I get why you’re upset, OP. We recently went on a family holiday with my in-laws and my sister in-law and her family. In-laws and sister in-law live 5 mins from each other, see each other daily and in-laws provide almost all childcare. We live 3 hours away. On the holiday we were staying in three cottages that were next door to each other and in-laws spent 90% of their time in sister in-laws cottage and rarely came to see us. We have children who rarely see their grandparents and who felt very unloved as a result. This kind of favouritism doesn’t go unnoticed and it’s very cruel.

I would ask your sister, had something similar years ago, turned out my family thought I was a bit overbearing. Hard to hear, useful to know. All sorted now.

I can see why you’re feeling put out. I don’t think it’s the fact that your parents are staying with your sister, it’s the fact that they’re not including you in their plans now. They must realise that you’d like to spend time with them all while they’re there?
Is your husband and other child in Spain with you too?

I don't think you are unreasonable and I think it's upsetting to be sidelined.

If you are close to your sister then I would talk to her rather than your parents.
It's likely her family making the plans, call her and explain that you were looking forward seeing her during her holiday, but it's difficult when you're informed of the plans so late.
Give her an opportunity to discuss what she wants from this time too.

If all you do is moan to your parents then indeed it comes across as a bit childish.