It's about the family logistics, so you should respond, in an hour or two. Just say "Thanks -- we'll be fine," and then do whatever YOU want to do that weekend. Make sure you do have something to do, and when you do, get her sister to babysit so that it's sure to get back to her that you went out.
DON'T say where you're going, but come back at a respectable hour (11:30ish).
