My brother has 4 girls.
The oldest niece is 13, my daughter is 17 but they get along really well (my DD is autistic and has a younger mindset)
If we are going shopping she often asks if niece can come along as they really like some of the same stuff and my DD only has a few mates her own age in college but doesn't like to socialise outside with them.
So DD asks if niece can come round for the day and I text my brother. He asks if we can take one of the other 3 nieces as they have started to notice its always the eldest that gets to visit.
But thats because my DD and niece get along and do stuff together.
The other ones are much younger, primary school age and DD wouldn't have any interest in socialising with them which means instead of me getting on with my day whilst DD and niece do whatever, I would actually be entertaining a small child, which isn't the same thing at all.
I know it might seem unfair that its always the same niece but its because of the age and dynamics of the kids friendship.
Now I don't know what to say as DD doesn't want to 'babysit' one of the younger ones today and neither do I.