Did David Marcus ever meet any of his cousins, such as Peter Kirk or his 2 siblings from "Operation - Annihilate!" ?? Were they even aware of each other, before or after David's death? Maybe Carol Marcus arranged secret meetings between them and kept it secret from Kirk, who was busy anyway. Maybe Kirk's nephews got in touch with David Marcus between TWOK and TSFS.
I doubt it. Come to think of it, I am not sure David even knew that Kirk was his father before ST II happened.
I'm sure he did not know Kirk was his father. Remember, he mentioned Kirk to Carol: "that over-grown boy scout you used to hang with". I'm sure he'd have said it a bit differently had he known.
Yeah, I'm reasonably sure that neither knew before TWOK. David seemed to know Carol used to hang out with Kirk, and Kirk presumably then knew that Carol had a son, but Carol presumably never saw fit to explicitly tell either of them until TWOK. Makes one curious to know if either of them (especially David) ever wondered, though...