Learn why "Coen brothers" is lowercase but "Andrews Sisters" is not.
For the Grammar Girl podcast about contractions in the Coen brothers movie True Grit, I had to look up whether I should capitalize "brothers" in "Coen brothers." The answer is no. It would only be capitalized if it were part of their official name--for example, if they were a band called The Coen Brothers. Here are some examples:
The Andrews Sisters
The Allman Brothers Band
Sources: Chicago Manual of Style, Official Coen Brothers Wiki
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