Edit: We already have Cousins in CAS. If you create an elderly sim, then two adults kids of that sim, and give each of those two a child, you'll see that the game automatically branded them cousins. So, you simply need to create the relatives that makes two sims cousins.
I don't expect them to have a common family tree. I think they said "cousin" as in saying they are not very close family (not sisters), but have the same family name. A cousin gets more vague as they don't even need to state whether it is via the father or mother.
I don't think EA should change family relations of sims that we have played since 2014. Many simmers made up their own stories, and I see it as weird if EA wanted to conflict with what simmers already set up. I never played the Goths but I'd be a little upset if they added changes to other pre-made sims that I do or did play.
Anyway, would be cool to learn their family history somehow :)
