Maybe the 4-year-old loves it? This might be his first quilt experience. As for your disappointment, it might turn into big trouble if you say anything to or about the stepDD.
Maybe give yourself couple years before mentioning anything inside your Family Circle.
Ask yourself, what is your motivation; what do you want to happen if you say you want the quilt back? Are you ready to accept responsibility for the unanticipated Consequences??