1. Mix pudding mix and milk at a low speed until thick.
2. Blend in the sour cream and half of the whipped topping.
3. In a trifle bowl or other large bowl, spread ⅓ of the pudding in an even layer. Add a layer of vanilla wafers, then top with ½ of the banana slices. Repeat the pudding-wafer-banana mixture.
4. Top with the remaining pudding, then spread the rest of the whipped topping on top. Sprinkle on crushed vanilla wafers.
5. Cover and refrigerate for at least 3 hours before serving.