One of the most fun things about younger siblings is how they refer to their older brother/sister. I'm told my brother gave up entirely on trying to say 'Kimberly' and renamed me 'Eh' (or 'A') for a while. =) It didn't last too long, but it makes for fun family folklore! (Love you John!)
We have always called James, James. When Joel was born we nicknamed him 'Joelie'. Once he could talk for himself (which was shortly thereafter =)) he started calling his brother 'Jamesie'. I guess it's only fair. This fit our African context well, because at the time we were living in a Bantu area where languages are mostly CV (consonant-vowel) and Joel became 'jowele' and James became 'jamese' (pronouncing every vowel like Spanish does). Even at 4 and 6, Joel still calls his brother Jamesie affectionately.
So, in steps Anna. Joel has always been very much a playmate as soon as she could smile and coo, so her first sibling referent was 'Dobey' for 'Joelie'. Joel loved it and I think it's cute. She can now also say 'Joel' (with most of the dark 'l'), but still chooses 'Dobey' or 'Dovey' [long o] quite often. Since James started therapy and relates much more happily with his sister now, she's dubbed him 'Namey', which must come from Joel's original 'Jamesie'. She's still honing her verbal abilities and can call James 'Jabey' now, which he likes a bit better than 'Namey'.
So for the time being we have 'Jabey, Dobey and Eena'. As linguists, we have so much fun with the process of first language acquisition, watching their mouths and minds working on language all day long! As I said, fun family folklore, at least to me!