When they arrived (Viv & Stefan) the story was that Stefan came to vivian, said he was her son, and thus Ivan or gus, whoever, had lied to her about the baby being stillborn, instead he gave it for adoption. That was HIS story (wasn't his name Max?) Anyway, supposedly DNA tests or something. But still......Vivian pulled Andre in on the con..........and when Abby & Chad wanted DNA and pulled Stefan's hair, it was Andre & Viv who took it for testing. I am of the opinion they switched the hair for one of Andre's. or something. They ditched Kate, who was supposed to be watching for hanky panky. Well, Vivian is dead, as is Andre, so "Stefan" has no one to dispute his claim to actually be a DiMera.
It is time to lay the DiMeras to rest, bring in a different villain & family. With Stefano gone, there is just no appeal for them any longer. He was the one with the "love to hate" charisma.