Calling sibling "Brother" instead of by his name
by Uchiha Sasuke
Question: My story takes place in an underground clan society. My main character has a brother that he is very close too. He almost always calls his brother "Brother" instead of "Lucas" (which is the brother's name.) He also calls his Mom and dad "Mother" and "Father" by the way. Is this really weird/odd, or is it OK? It would feel weird for me to make him start calling his brother "Lucas," since in my mind its become part of his character that he addresses him as "Brother" (in his mind, and when he is talking to him. When he is talking to other people about his brother, he says "my brother")
Since you're writing about an underground community (whether you mean under the earth or just below the radar), it would be perfectly reasonable for them to have some idiosyncrasies. Since you are designing the community, you are free to decide on its unique customs. There are some communities where people use different forms of address, sometimes just because the community evolved its own customs through isolation from the rest of the world and sometimes from a deliberate desire to set themselves apart from non-members. It's similar to communists calling each other "comrade," members of a religious group calling each other "neighbour," sailors calling each other "mate," or men in Newfoundland referring to each other as "buddy."
As long as you are consistent with your rules and human nature, you can give your community, or even just one family, whatever customs you like.
(Another example, from literature, is the book A Clockwork Orange
in which the main character addresses the reader as "Oh my brother.")