“Cersei has no claim to the throne and Westerosi laws of succession (outside of Dorne) favor males over females.”
Cersei has a claim as wife of Robert Baratheon. The laws favor males over females, certainly, but they don’t preclude females if there are no males left to inherit (just look at Lady Lyanna Mormont, a nine year old girl who inherited the lordship of her house).
“So, theoretically, Jamie would have a better claim to the throne than she does.”
Jaime is a knight of the kingsguard so he forsook any claims he might have had, which would have been pretty weak to begin with, as a brother-in-law on the female side.
Good points. However, Jamie is no longer a knight of the Kingsguard. He was removed by Tommen. Not sure what happens now, but I would think that he could inherit titles and lands again (if Cersei allows it).