Combine the macaroni, broth, and butter in the Instant Pot.
Secure the lid and set the Steam 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 Steam 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.