Step 1: Give Them an Unusual Name
You can start by giving your sibling a name that's clearly not their real name. For example, instead of "Bob," try "Bizarro McSnazz" or "Bubblepoof."
When they ask why you gave them this name, just shrug and say, "I was trying to be unique."
When they ask to change their name, just say "No."
As time goes on, they'll start to resent you for this decision. And that's when you know it's working!