In a large saucepan, cook rhubarb, strawberries and 1 cup sugar until fruit is tender. In a large bowl, dissolve raspberry gelatin in boiling water. Stir in fruit; set aside.
In another saucepan, heat milk and remaining sugar over low until sugar is dissolved. Meanwhile, soften unflavored gelatin in cold water. Add to hot milk mixture and stir until gelatin dissolves. Remove from the heat; add vanilla. Cool to lukewarm; blend in sour cream. Set aside at room temperature.
Pour a third of the fruit mixture into a 3-qt. bowl; chill until almost set. Spoon a third of the sour cream mixture over fruit; chill until almost set. Repeat layers twice, chilling between layers if necessary. Refrigerate until firm, at least 3 hours.