(Denny Crane: "Sometimes you can only look for answers from God and failing that... and Fox News".)
I don't think it says in the article that they didn't know they were related, they knew and had a relationship anyway if I read it correctly...
From the article, it seems like they didn't know each other until 18, but they did know they were related to each other.
How they found out they were related or what the circumstances are, I'm not sure, but obviously, they did know.