A few years ago I did the ancestry.com test and found out one of my father’s affairs produced twin girls. When my half brother took his test it turns out he was actually my full brother. Seems my mother cheated on her boyfriend and slept with my father, who was also her husband at the time. DNA testing can reveal all kinds of things!
When we did the Ancestry thing, I finally found out who my biological father was. Never met him, he’d already died, but I had a name and a whole slew of half-siblings.