I suspect that it's not them organising things to leave you out so much as sibling 1 saying "I'm going to X, anyone want to come" and those who have children of similar ages think "oh yes, mine would like that too" whereas you hear it as "we're choosing to go deliberately to something you can't go to."
Finding something that teens will find interesting and toddlers will be able to access is pretty hard. There's 7 years between my oldest and youngest and it's hard enough to find something that they'll all enjoy.
My teens love playing with younger ones, and would have been very happy to look after one of yours for a couple of hours. But not all day, nor two children, when they're on holiday.
I wonder whether they are feeling expected/pressured by you. That's actually really off putting. As I said, my dc love their younger cousins. When we're together they play with them, look after them-as I did with mine.
But there is one set of cousins whose parents have made it very clear that they are only interested in them as babysitters/to look after their dc while they gossip. It's a real turn down for them, and it really does mean that they don't enjoy being with them because the expectation is that they do all the looking after. If I say we're going round there, the chances are all three of mine will opt out, whereas the other cousins they'll be champing at the bit to go.
I have also been on the older side of this. Never again. We had children aged 2 through to 9. The others had a baby apiece. Because we had older ones who were more self-sufficient, we ended up doing most of the catering (self catered) a lot of the housework and a certain amount of the babysitting. Even going out for the day was a pain. We'd say we were going out to this place, others would decide they were joining us, and then we'd either be expected to fit round baby routines (and one was very rigid), not stop to do anything that was age appropriate for our dc, and we'd be left to do any carrying/fetching/general work as neither of the couples "could" as it took both of them to look after one baby..... For example on the beach they all decided it wasn't suitable for babies (fair enough) so spent the entire day in a café leaving us with all their things-and they'd packed as though they were expecting to camp for a week.
They'd literally just dumped the stuff by the walk down and left us with it.
I have said never ever again! And dh knows I mean it!
I think there is an aspect that when your dc are pre-schoolers it's very easy to look at parents with older children and think they have it easy, and have far more time to do things etc. Now I'm at that stage I know a few things.
Firstly, I love toddler stage etc. Before I had children I I had infinite energy to deal with them, loved playing with them-would go out of my way to deal with babies/toddlers. I still love them, but I don't have the energy or inclination now to do the full on play. I can if I have to, but wouldn't choose to. I've done that stage.
Secondly, parenting teenagers is hard. Far harder than the primary and below stage. You can't always kiss it away and hug it better. They need you just as much but in different ways.
Thirdly, teenagers have their own opinions and you can't tuck them under your arm and continue. We're going to a petting farm. "Wa! I don't want to" says the 3yo. "Tough you're going anyway" you say as you strap them in their car seat. They'll enjoy it when you get there. Teenager will say "boring, I'd rather stay at home and play on the x-box" and if you insist will remind you that at intervals. Moaning from a teenager who told you they didn't want to go is far more irritating than a toddler-and they probably won't sleep in the car on the way home to give you a break either!
On that note: Teens also need to have days out they enjoy. It's a difficult age, too old for petting farms and not old enough (generally) for walking round National Trust gardens. What they like is often more expensive than either too! Just as I would opt out of walking up Snowdon as I don't like hill walking, they know what they like and will have an opinion. And express that opinion.
Next: Bribery is great. I love it. However: What works for a toddler does not work on a teen. My dc as toddlers could be persuaded a day was great with the promise of an ice cream at the end of the day. That doesn't work on a teenager. They'd laugh at the idea that a day of being bored was worth an ice cream at the end of it.
Also teens need their parents. They might not like to admit that, but they do. They also need 1-2-1 with parents. That's what holidays are for. Mine all love to have time just with me or dh.
Lastly: Teenagers need their holiday times. They need to do different things; they need to have space. Yes, they can, and will help with chores. They don't need constant supervision to do said chores. But they do need time to do what they want without the expectation that they are anything other than children. No responsibilities, no time pressure etc.
You'll be looking at this list as a parent of pre-schoolers, and thinking "really?" I suggest you save it and come back and look at it again when yours are 16 and 13yo and see if you agree. You'll find your thoughts have changed. 