Preheat oven to 350 degrees. In a small bowl, let the raisins soak in the brandy for 20 minutes. Grease a shallow baking pan with butter and layer sweet potato slices. In a saucepan, combine brown sugar, cinnamon, water and margarine and bring to a boil. Add the raisins and brandy and pour mixture over sweet potatoes. Bake uncovered for 40 minutes, basting with juices.