I always write on the invitation if siblings *are* invited. In my circle, it's pretty common but not always. Sometimes when you call to RSVP, the hostess will say, "Oh, and you're welcome to bring the other kids if you'd like to!" But yeah, if they don't say anything like that, I think assuming that they're *not* invited is the way to go. I think the clearest way to address it is to put the invitee's named on the front and ask people to RSVP "because we need an accurate head count--thanks!" or something like that.
What do you guys think about being more specific (when they call to RSVP) for known offenders? For instance, you invited Sally, and you know that last year, her siblings Rob and Mary were dropped off with her when you invited her. This year, when Sally's mom calls to RSVP (okay, I'm optimistically hoping she'll call), can you say, "Oh, and by the way, this year the invitation is really just for Sally, as we have a very strict number of spots. We've been having to tell people that we can't include siblings because we just don't have room." Or something?