Hey Moonsanata! Have you read the script for the possible Borgias film? Neil wrote it for the fans as a better ending to the story. Maybe you should read it? :)
Well he did marry Sancia and moved to Naples. But that is no reasons for cutting ties with your family. Especially in the time the show is set in and how family kept close back then to remain strong for the sake of politics. In my opinion, it's most likely because the writer got lazy, especially when Joffre isn't even mentioned in the dialogue. Other missing characters are for example Giulia's husband who she mentioned in her first episode.