Combine broccoli florets in a large mixing bowl with the red grapes and red onion.
In a small bowl, stir together the mayonnaise, sour cream, vinegar, and sugar. Pour dressing over broccoli mixture, and toss until well mixed. Cover the bowl and refrigerate the mixture for several hours or longer.
Just before you are ready to serve, stir in feta cheese crumbles and sunflower seeds.