Mother's Day is a day to express appreciation for your mother or other mother role model in your life. I think it's a heartwarming gesture that you want to express your gratitude for your stepmother. As a stepmother myself, it's always a delight to have breakfast or brunch on Mother's Day and spend some family time together. But if you are not able to be with your stepmother on that day, I think flowers and a card would be greatly appreciated. If your budget allows for more, perhaps you could think of arranging a time for an outing together. Maybe tickets to a favorite play or concert, or a movie on a Saturday afternoon. Whatever you choose, I know that she will be delighted that you took the time to recognize her efforts.