Cersei is just a pretender to the throne, right? Not according to Reddit user webby131, who did the hard work of mapping the Baratheon bloodline back far enough to find that the only living relatives of Robert can be found in House Lannister. Now, since Tywin is dead, Jaime is technically the rightful heir to the Iron Throne. Remember, Tommen relieved Jaime of his status as a member of the Kingsguard before his death, which means Jaime can now inherit lands, marry, and do anything else he wants to do. The one thing he doesn't seem interested in is ruling the Iron Throne.
So, let's say Jaime ignores his claim, or is unaware of it. In that case, Tyrion, as the second son, would be in the next in the line of succession — well, he would be if he hadn't been convicted of killing his nephew, committed patricide, and joined forces with Daenerys as her Hand. That means that by default, Cersei is the rightful heir to the Iron Throne. She's not a pretender at all; the rules of succession point to her, as long as her brothers stay out of her way.
