This is a hard one either way. With young children, generally mothers are expected to stay at birthday parties and help supervise their children. This brings up the obvious question of siblings...many families can't afford to hire a babysitter to watch one child just so the mom can attend a birthday party with the other child. Frankly, in my experience with parties we've given, it's been harder when moms just dropped their kids off and left to go shopping or something than when they've brought a sibling. And I've been the one in the position before where I've had to RSVP in a qualified manner: my son would love to come, but I will have my daughter with me at that time too, so it may not be possible unless I can bring her or unless I don't need to be there. Then I've tried to leave it up to the mom whether I drop my son off or stay and bring my daughter.
I have also observed that it is often possible to bring a sibling into one of these parties without the hostess having to pay, so long as the second child does not participate in the key activity (for example, bowling, if that's where they do the headcount, or sitting down to eat, if that's where they do the headcount). I've done that with my daughter a lot with parties where my son is the invited guest. She just stays with me and isn't counted.
So, on the one hand, your guest may not have been the most tactful in HOW she asked the question, but the good side is that she probably takes her responsibility to stay and help seriously and that's probably why she asked. Does that answer what you do in the situation? No...there are a lot of variables to consider. But perhaps the above will help you open a discussion with her about options if the cost is an unavoidable issue.
B.