I think you were both bu. Her for expecting you to be in a certain spot even though you couldn't get through to her and didn't know if she'd got the message or how long she'd be. You for speaking about her family choices in relation to her son, whom you see as the golden child when you knew the situation was already tense.
Dh and I vowed never to go away with brother/sil and their son (6) again after two lowsy times where they were awkward and overbearing and treated us and our dd pretty poorly, the first time dd was 6, the second time she was 7. As it happens we are now no contact with both of them after their appalling behaviour earlier this year. Ie sil screamed at dd (8 at the time) on the day of dds grandfathers funeral (my stepdad so not part of her family) and brother physically abused me at the burial of stepdads ashes. Dd was also present and she's shit scared of both of them. As am I. I'm chronically ill and disabled through the illness.
I think you should take a big step back and see what is really going on. My brother and sil have always expected us to sing to their tune, adapt plans on their say so and be able to do what they want to do even if I'm not physically able as they deny I'm even ill or have chronic pain. They are functioning alcoholics
If your sister is like this, I can imagine you're annoyed. However, if she's like this to accommodate the actual needs of small children, it is totally acceptable and you cannot all be expected to be together for the entire duration of a short break with much older and active children as the two are incompatible. If she can't see that, she's still living in baby land.
My advice would be to not go away with them again. There's something not right with the dynamic. I don't know if you're attributing to it or if you're refusing to cow tow to unreasonable demands, which is causing drama.
I really don't like the talking behind other people's backs. That is what brother and sil and mother do to me. Constantly. In a derogatory way. Denying my illness, coming to conclusions I wouldn't act in a certain way if I were actually ill and stating my dd is going off the rails for totally age appropriate behaviour. I, of course, was totally unreasonable with my brother at the burial. It is only to be expected that he physically abused me because I shouted at him. The fact that he shouted at me first "you're not disabled" when I told him for the fourth time I couldn't do something he was repeatedly telling me to do when I said I couldn't as I wasn't physically able. He is under the childish belief I was being awkward. No dickhead. That's your role.
What I'm saying is. Look at your sister. Really see her and her family. And then decide who's contributing to the poor dynamic.