Mind you, Carolyn did ultimately come across to me as somewhat self-delusional.
She would have to have been to have married Jeb.
I am rewatching the later half of the Leviathan storyline. I am watching the episode where [spoiler] Quentin tells her Jeb killed her father and she gets all defensive, accusing him and Barnabas of pointing the finger at Jeb just because they didn't like him. I wanted to smack her. She's only known Jeb for what, a week or two at this point, not to mention she had that dream of him where he admitted he killed her father. All right, a dream doesn't count as positive proof, but she's known Barnabas a lot longer than Jeb. She hardly knows anything about him as a matter of fact yet she jumps to his defense because Quentin tells her something she doesn't want to hear. She won't even consider the idea. [/spoiler]
Back to topic, as far as Carolyn and Vicki went about forgetting what Barnabas was....I suspect Vicki may have chosen to live in denial about him.
When she was in 1795, she is told Barnabas [spoiler] has died instead of going to England. Then how could the Barnabas of 1967/1968 could exist if his "ancestor" didn't live long enough to sire some offspring? [/spoiler]
As for Carolyn, maybe she just didn't want to expose Barnabas since they were 'blood' (albeit a few generations separated them) and she still felt some loyalty to him. Or maybe she just pitied him.