Honestly, if one of my boys comes home with a party invite, the other doesn't go unless it's stated on the invite! At the end of the day, the family is hosting and they set a budget for a certain number. For DS1's 5th I think we had 20-30 kids. He had 10 from Kindy and it was stated on the invitation that siblings were not invited as I had only catered for x amount of people - the other children being relatives and close friends. I think that if you state that siblings are not welcome, then that SHOULD be resepcted.
As for these older kids who treated the little ones badly, took advantage of a pony ride that was not for them and eating the party food, that is NOT okay. If they were teenagers then the parents should have told them they need to go do what teenagers do and stay away from a kids party. And for the parents to NOT deal with their brats is just disrespectful! You handled it far better than what I would have. I would have not been so polite!