In a large heavy skillet cook the apples in the butter over moderate heat, stirring, for 3 minutes, or until they are softened. Stir in the lemon juice, the brown sugar, the cinnamon, and the Sherry, bring the liquid to a boil, and simmer the apples for 3 minutes. Add the sweet potatoes and cook the mixture, stirring gently and trying not to break up the sweet potatoes, for 2 minutes, until it is combined well and heated through. Transfer the mixture to a heated serving dish.