Sprinkle gelatin over rum in a small saucepan; let stand for 5 minutes to soften gelatin. Heat, stirring constantly, over low heat until gelatin dissolves, 3-5 minutes. Remove from heat. Pit and peel avocado; cut into quarters. Puree avocado and yogurt in a blender bowl. With blender running, slowly pour in gelatin mixture and continue to process until thoroughly blended. Transfer to a large mixing bowl. Beat egg whites in a mixer bowl to soft peaks. Gradually beat in sugar and continue to beat until stiff. Gently fold egg whites into avocado mixture. Spoon into serving bowls or 6 goblets. Refrigerate covered for 2 hours. Sprinkle with Pistachios just before serving. 167 calories per serving.