K.C.
I've never understood why so many parents think that an invitation for one child is actually an invitation for the whole family. My siblings and I never attended the parties of friends when we were growing up. But I know it happens a lot these days, so you're right to worry a little.
Send paper invitations in the mail and address each one specifically to the child you are inviting. You can put this on the invitations:
"We hope Chris can attend. Please RSVP by Jan 31."
If anyone asks specifically about the siblings when they RSVP, tell them they are welcome to bring them for $8, or tell them that, unfortunately, the party is just for the 2nd graders. It's up to you to decide whether you even want the extra people there at all. If a sibling does come, require that the parent stays too. You shouldn't be responsible for their extra kids.