[M4F] Surprise Christmas Gift

Premise: A single mother has two children, her oldest being her teenage daughter (15-17) and her youngest being her son (12-13). Well, for one Christmas her daughter approached her with an idea to make a Christmas that they would all remember. It took some convincing, but the mom eventually agreed. Christmas came, presents were opened, and the mom told her son that she and her daughter had one more gift for him. They left to get it and were gone for several minutes. When they came back, they were wearing nothing aside from plastic bows on their nipples and very thin ribbon fashioned into panties with mistletoe hanging off the front. The mom tells the son that she wants him to try to impregnate both her and his sister. "Give us a very white Christmas"

So you'd be playing both the mother and the daughter. Whether you want them to try to compete for the most attention from the boy or have the mother try to get her children to have a special relationship or have the daughter try to see her mom's enjoyment from making the boy the man in her life or even some entirely different goal is up to you.