[P.fitz notes: Hi Ran I asked GRR about the civil war "The Dance of the Dragons" why the Kingsguard had split, if whole Kingsguard had split between the two factions or just one of the Twins who took a opposing side to the rest of his brothers ("Ser Erryk and Ser Arryk who died on one another's swords..." p 65 AGOT Voyager paperback Bran's second chapter).]
The Kingsguard split. At the times, the laws of succession were less well established, and there was dissent as to which claimant had the best claim, the elder daughter or the younger son.