Ok I just read the wikipedia of King Robert and it explained how he became king and his family background. But it does spoil one thing that will eventually be revealed in this series. So if you dont want to know then I guess dont read. But channces are you either had your suspicions or knew already.
Even tho I read the spoiler I'm not totally shocked given what I've seen after 3 episodes. Just uhh well keep my comments to self.