It can be awkward but it doesn't need to be. I think it's important to be gracious to the stranger perhaps by responding, "I'm blessed to be his stepmom." Or perhaps no response is necessary, just a smile. I think it's important for stepchildren to know that you are accepting of your place in their lives no matter what others assume it to be. Be proud of the relationship. However, it may provide the perfect private opportunity for an open discussion with your stepchild to see how they would prefer you respond. Getting the child's feedback can be invaluable in this type of situation. Then, the next time you hear this question, you both will be prepared.