Yeah, DD12 always agitated for a sibling as well. When she was younger I'd just tell her that it wasn't going to happen. However, one time recently, she was whining and I just told her bluntly "if you wanted siblings, you should have slept more as a baby."

Which is true by the way. We had planned on a second, but after getting her to finally sleep through the night (when, and I kid you not, she was like five years old) we knew we were done. DD is the best ever, but she took a decade off our lives as a baby and toddler.