Combine the macaroni, broth, and butter in the Instant Pot. Secure the lid and set the Pressure Release to Sealing. Select the Manual setting and set the cooking time for 6 minutes at high pressure.
Perform a quick release by moving the Pressure Release to Venting. Open the pot and stir in the cheeses, sour cream, mustard, and cayenne pepper. Let sit for 5 minutes to thicken, then stir again and serve.