Nah, you wouldn't get a dozen or more egotistical princlings living in a castle or city with a father who disappeared for years at a time without most of them making their own changes to the design. The Castle would have a central area built by Oberon, probably practical and utilitarian and then it woudl spin out to wings and additions in all sorts of styles, a cluster of Italinate courtyards, a handful of minarets, etc etc. The City woudl be even more diverse.
As for the Amberites. they really have to be diverse don't they or it simply makes no sense that they are the base shadows. I agree that those with shared mothers need to look alike. I can see Gerard, Caine and Julian as Arabic or African; Corwin, Eric and Diredre as Hispanic; Making Benedict Oriental is too obvious but giving him a native American look works; Random needs to look like Christian Slater in 1995....