I name changed for this, as it's very identifying.
I have 2 sisters, my older sister has gone through a really rough summer, she has one nine year old and he's never been abroad before, he's only recently received his passport.
My younger sister has 3 kids, between the ages of 10-5 and they go away every summer.
DH and I have decided to go away for October half term, we only have DS (6) and we really want to invite my 9 year old nephew, but knowing my younger sister, she'll be upset with me.
I don't think it would be fair to invite one of her children and not the other two and we can't take them all.
I don't think I'm being unreasonable but I'm not sure if others would view this as unacceptable.