I think if the family was willing to squeeze in together, Disney probably and should allow the "group" to stay together. I mean how unfair could that turn out to be? A family of 6 - mom, dad, 4 kids? Oh sorry, one of you can't stay in the castle?? How wrong would that be? Very "un"magical.....
It probably says 5 because that is the basic family size - 2 parents, and up to 3 children... or say 2 adults, one child and grandparents, etc.....
They obviously don't want you inviting say all 10 of your friends and family to stay....especially if you lived close by......
I guess the say..."There is an exception to every rule" applies here....