brother would have had to derive from monday, so it should have the same connotation.
yes but since it's derived so far the meaning of brother is completely different from monday. However, if you use brother in a tribal context it can still be considered offensive. Examples:
"ayo wus good my brother how's it hanging" - not really offensive since 'my brother' is meant to represent companionship, even when exchanged between people of different races
"hey man whats good my monday" - somewhat offensive but some people may let it slide
"all brothers deserve to be lynched" - blatantly offensive.
Think of it as if you have a crippled friend who is terminally ill. You have a nickname for him which you constantly use as a teasing joke but he's alright with it and doesn't take offense from it. Say that nickname is 'cripples.' If you call him by his nickname 'cripples' he will probably take it as a joke. However, if somebody else who doesn't even know him walks over and says 'hey cripples' he may take offense to it or feel bad because he doesn't feel comfortable hearing it from other people.
Applying that logic, it is a clear fallacy to say 'well black people can say it to each other so why can't we' because, for obvious reasons, it's offensive.