Place macaroni, broth, shallot, mustard, nutmeg, cayenne, salt, and pepper into Instant Pot and stir.
Set vent to sealing position. Cook on manual mode, high pressure, for 4 minutes. (Pot will take about 10 minutes to come up to pressure before timed cooking begins.)
Carefully do a manual pressure release by turning the vent to open. (We like to use a towel or a long-handled spoon to do this.)
Stir in milk and cheeses until cheese is melted.
Stir in sour cream.