Would you?

[Trillian]

I don't think it would be an issue - I assume that the cousin is a fair bit older anyway if you see your Aunt and Uncle but not your cousin? In a large family names are bound to overlap a little sooner or later.

ETA - I forgot to say too that a lot of those types of names can have several shorter versions - a name like William for example can have either Bill or Will as the short ones, same with Robert having Rob or Bob. With girls names a name like you suggested Annabelle, could be Anna or Bella/Belle. So if you are likely to shorten it anyway you could always choose the alternative short version.