"What you don't do is take one child away, without their sibling, and then decide NOT to take the younger one away without their older sibling! for fear of upsetting the older one. Does that make it clear enough?"
^"It is not clear enough. Where are they going? What are they doing?
Maybe somewhere where a particular 4yo would be too much hassle? E.g. too whiny/fussy/misbehaving to enjoy something particular. I don't know, just guessing."^
I will try again, Nataleejah.
I am not saying it is wrong to take one child away on their own, if the activity is unsuitable - or if you want some time with just that one child.
But it IS wrong if you don't subsequently take the other child away for an equivalent treat, on their own, leaving the sibling behind - because you don't want to deal with the first child tantrumming about the second child getting a treat.
You have to treat both children fairly. That doesn't mean they have to have the same treat, but it does mean that one child shouldn't get better treatment than the other because they kick up a fuss, and the parents don't have the balls to face that.
If one child gets a treat, the other one should get a treat too - ie. if one child gets taken to see a film that is unsuitable for the younger sibling, then the younger sibling gets a cinema trip to a film they want to see, at a later date.
It means that both children should get to have trips away with their parents and without their sibling - maybe different types of trip to suit their ages.
In the OP, the niece gets to go away with her parents, but her brother does NOT get to go away alone with them, because of the fuss his sister makes. That is not fair.
I really hope this makes things clearer.