Prepare pudding mix according to package directions, using a whisk and 2 cups fat-free milk. (Do not use an electric mixer.) Add peanut butter and sour cream, stirring well with a wire whisk. Set aside.
Line bottom of a 2 1/2-quart casserole with 14 vanilla wafers. Peel and slice 4 bananas. Top wafers with one-third each of pudding mixture, banana slices, and whipped topping. Repeat layers twice using remaining wafers, pudding mixture, banana slices, and whipped topping. Cover and chill at least 2 hours. To garnish, peel and slice remaining 2 bananas; arrange slices around outer edges of dish.