J.D.
As a parent I would rather know ahead of time of a special b-day party. so I would be upfront on the invite about the cost and sibling issue. I would even type it out on a small piece of paper and insert it in the invite. I would just say that the child invited is paid for and everyone else is not. any extra tickets and food is on the adult to pay for.
They only other way is that she gets to invite 2 or 3 friends and you take them alone. She will have fun wether 10 friends are there or 2.