The writers are acting as if Alexis had no clue about her father when she knew him as a child. He did not have a ONS with her mother, he had a full blown affair with her for years. In fact, Alexis was a pre-teen when her mother became pregnant with sister Kristina. It was upon learning that her husband had conceived another child that Helena went crazy and slit the woman's throat.
Alexis heard them arguing and hid her little sister. She returned just in time to watch Helena do the deed herself. The house keeper took Kristina to raise and Mikkos took Alexis. Helena refused to allow her to be called his daughter so she was introduced to her brothers as their cousin. Even when Julian was pronounced to be Sam's father, Alexis told him that her father made her give up their daughter and transferred her back to Europe to finish school. After his death she was left with Helena who could barely stand the sight of her and agreed to send to any college that was far and away.
Stefan discovered the truth that she was really his sister and welcomed her as such. And when a grown up Kristina came to town, Alexis was worried about Helena finding out the truth of who she was and killing her. Luis beat her to the punch. So all this madness of her and needing to rely on Valentin to find out about her father is hog wash. Another rewrite that doesn't hold up to history.
Writers, if you wanted to find a way to bring the last standing Cassadines closer why not simply use Charlotte? She is after all Luke's grand daughter, making her like her younger brother related to Michael who Sonny's adopted son who Krissy's cousin who is Sam's sister who are both Alexis' daughter's and cousins to Charlotte as Alexis is her aunt by blood. I mean really, could you have not come up with something better than little brother suddenly needing help from his big sister who has shot him down so many times before with regards to legal advice. Now she needs to know about their father, a father that he never spent one day under the same roof with, but suddenly has all this worth while information on. Get Real