[Opinions] Re: Adults with "childish" nicknames
Yes by what you've stated, you did not honor your brothers intent & continued to call him by your greater or more natural association--even familiarity--although this disagrees with your own idea. The "childish" portion i.e. "ie" or "y" soften the formal edge.
I remember you've stated that you are not a fan of your name, yet that you'd never change it due to respect. I hope you play with the idea of shortening your name to the first syllable--then adding an "ie" to the end, not necessarily legally, but just as a pet name among friends--perhaps introducing yourself with this new informality. This is not a choice that needs an immediate decision. Whether you do or not is only up to you, as it only concerns you; yet if it does not resolve the complications you reference in your last sentence, it may just add harmless enjoyment for someone who simply loves names.