There are no occasions when she has to curtsy to Catherine (and William is on record as wanting to do away with bowing and curtseying). What we will see, if they attend the service at St Paul, is Harry's wife make a big display of curtsying to the Queen and Charles (but not Camilla).

Hoping the seating gets assigned so that Harry and wife are seated across the aisle from the Queen, Charles and Camilla, and the Cambridge family. That would make it impossible for them to get a money shot with the Queen.