Me hating Fiona is incidental. This is about the story not making any goddamn sense.
This. I really like Fiona (not my favorite, but I like her), but it is irrelevant in this discussion.
I think I am starting to understand a bit now, though. They told him Maric was his father because Maric wanted him to know; he didn't like the idea of Alistair not knowing who his real mother was, so he probably wanted him to know who his real father was.
The following is all a theory:
For his mother's identity, they chose a real person. Some human woman was pregnant by a man who wasn't her husband and then died in childbirth around the time Alistair was born. Maybe she had a stillborn, maybe not and the kid was given up for adoption; doesn't really matter. Point is that a real person was chosen to be Alistair's fake mother. Because of this, he was able to look up information on her and find out that she had a daughter named Goldanna, who he assumed was his sister and later sought out. The people in charge of this big cover up might have suspected something like this would happen, so they lied to Goldanna, gave her some hush money, said that her mother had been impregnated by King Maric and that both she and the child had died.
This, or they chose a random woman/girl (depending on how old she was at the time) who they could trust, or who they could bribe to keep quiet, and told her to pretend all of this. Then it was her own decision to pretend to resent Alistair in an attempt to guilt-trip more money out of him.