Aunt and uncle take 9-year-old niece to Disney World, refuse to take her 12-year-old sister: 'We simply cannot afford to take all of them.'

They should either take their daughter alone or both of their nieces. Taking one niece but not the other is just plain wrong. 

There are life circumstances where parents let one kid travel while the other stays at home. If their daughter has a dance competition across the country, it's going to be a lot more convenient for mom to travel with their daughter while dad stays at home with their son. There's a chance their son might be sad that his sister gets to go on a plane and he doesn't, but it's much more likely that he'd be relieved he doesn't have to sit through hours of tap, jazz, and ballet. They might also let one go on a school trip while the other stays home, but there's a good chance the 8th-grade trip to Washington, D.C., will still be around by the time their 6th grader gets to 8th grade. It's not like the younger sibling is getting excluded from some once-in-a-lifetime vacation that they'll never get to experience, nor are they losing out on valuable family time. 

It's a good rule of thumb never to exclude a child from a vacation that their sibling gets to go on. If they're a baby that won't remember it, that's fine, but if grandma and grandpa go to the beach with their 7-year-old grandson and not their 5-year-old granddaughter, she will probably be a little resentful of that for the rest of her life.