"I get the impression that they don't like going away on their own at all".
Absolutely. And that has to be why they are trying to muscle in on a holiday that you have already booked. You have planned your holiday and it has been planned to suit just the two of you, otherwise you'd have been discussing it with others before making any decisions! If you are not firm - and stick to it - you will have your holiday ruined. And that would be ridiculous. Hope you can see that.
Shame you don't have surprise or indignation from your DH. And I note that not once in your post do you refer to them as 'friends'. I can't imagine anything worse, I should be seething that they feel they can just ride roughshod over arrangements.
It's often hard for people to be ASSERTIVE ! But that's what you need to be. You don't need to agonise over something that is entirely your affair, funded and organised by you, planned for the two of you. All you need to say is 'sorry, not this time - we've booked a holiday just for the two of us.' If she doesn't like it, then that's tough. No discussion, your decision. You are being disrespected if you have to explain or compromise on your own life plans by those who clearly depend on others for the entertainment and distraction they can't find in their own relationship.
Do it this once, OP. And don't feel bad. You won't be agonising in future. This is the way to start having a much better life and enjoying something for yourself. You will be replaced by others they can take advantage of, so go for it. Who says you can't do it? Only you!