"gotdamn there are so many forgetin brothers on my plantation" (voiced by redneck white man in a demeaning tone)
saying 'brother' in a southern white accent is like using 'maths' in american english instead of 'math'
it's really rare to hear anyone use brother as an insult, even when white people say 'brother' it's usually accepted in some parts, especially if they're close friends of yours. of course if you say 'i want to lynch some brothers' it's tribal but it's rare to hear that. 'i want to lynch some mondays' would be more common.
They can be used interchangeably but that's rare. Also, in terms of context, nobody would be caught dead saying 'monday' in an endearing way. 'whats good my mondays' or 'hey you're my monday' both sound extremely horrible and insulting, compared to 'whats good my brothers' or 'hey you're my brother'