“She isn’t in the Bloody Line of Succession!”
True, but I think everyone line of succession is dead. Joffrey, Tommen and Renly had no offspring, Robert had no legitimate offspring, Stannis only had Shireen and she’s dead too.
As for other Baratheon cousins, all the brothers of Robert are accounted for with no living heirs and we don’t ever hear that he had any sisters, so I think that line is dead except for bastards like Gendry.
Besides, Cersei is determined to be queen no matter what the line of succession says and she just killed everyone who would have objected about it in King’s Landing :)
Robert Baratheon became king in part by right of conquest but also because he had the best claim to the throne (not counting Viserys and Daenerys), because his paternal grandmother was a Targaryen. That was how they justified it.
Cersei has no claim to the throne and Westerosi laws of succession (outside of Dorne) favor males over females. So, theoretically, Jamie would have a better claim to the throne than she does.
Cersei was married to the king and mother to two kings, but she doesn’t have a claim on her own. But now she’s killed off most of her opposition (or so she thinks), so there’s no one left to object.