Even if it were allowed for a Kingsguard to defect and fight on behalf of the accused, Tywin would never allow it in this instance. Jaime is missing a hand, significantly less skilled as a swordsman than he was before, and would mean Tywin would be losing probably both of his heirs at the same time (Jaime loses, he and Tyrion both die). End of Lannister bloodline.
I thought it was interesting that Tywin subtly acknowledged Jaime's and Cersei's incest-born children in this episode.
Nice that Oberyn was asking the hard hitting questions on the trial.