If it's a party where the parents are expected to stay, then yes, you should expect that siblings may end up coming, too, if the other parent is busy at the time. I was never in this position because my kids are far apart in age, but if I had had no one else to watch my other kids, and the hostess asked me to stay, I would have expected to pay the fee for the siblings myself. I would, of course, have called you first to be sure it was OK, and send regrets for my invited child if it wasn't.
If your experience tells you that the group you have invited is likely to show up with siblings in tow, then perhaps you would be better off adding a gentle discouragement for parents staying. This should be done on the invitation: Outline how many adults you have available to supervise (with a party of 10, I'd say at least 4) and make it a point to make the end-time firm on the invitation, and also state a specific location for picking up the children. Give 2 phone numbers for yourself and your child's other parent, and invite them to call ahead if they have questions.
It might even be helpful to prepare a short list of places of interest near the venue if any of them live further away, so that they will know what places are nearby where they might take the siblings while the invited child is at the party. I wouldn't send that with the invitation, but having it ready to offer when they tell you that they don't have a sitter might help. (Parks, bookstores, libraries, skating rinks, branches of the YMCA, etc.)