Well I suppose they could have, even if Angoriel is a half-breed and far from a first son, let alone first child (his father was Samael, by the way; Uriel was only his lord and master). It would also pretty much have required that CeCi had gone on a trip to Urbaniel at some point, since Angoriel never left there until he went in search of the Carnival. Which I guess she might have?