2) Watch the scene where Ravenna kills the king and tell me it wouldn't work if someone auto-corrected the names to Cersei and Robert.
4) Both ladies have no problem bathing in front of their twin brother.
5) Both brothers will eventually be attracted to women a bit more... militant.
8) Neither of them can bring people back from the dead. Yet.
And a few reasons why they're not:
1) Jaime Lannister has amazing hair. Finn... not so much.
2) Cersei alienates her allies and appoints dangerous people to power because a) she's arrogant; b) no one ever taught her how to rule a country.