Something to consider ..
The Brothers Grimm refers to the "unit", the "act", the "legal (so to speak) entity", the "artistic entity". You are referring to the "group" as one entity.
It's like saying "The Beatles".
Conversely, referring to the Grimm brothers is like saying "John, Paul, Ringo and George" or "all the people in that room" or "everyone here wearing blue" or "the list of people I iterated earlier".
Of course, this is not hard and fast. There is plenty of cross-over and mixing here.
As an "act", a legal unit, an "artistic unit", we typically say
I think really just because it sounds cool and someone started so doing and it stuck; and as an actual legal unity, or an act, we say
because they happened to explicitly choose that as their "act name" (rather than say "the Siblings Carpenter" or many other choices).
But, it is the case that "The Brothers Grimm" refers to "a unit" (You're saying "The Beatles", not "John, Paul, George and Ringo".)
Any form "The Brothers X" does tend to lean, perhaps absolutely leans, towards meaning the unit. The other way around, tends to mean shorthand-for-listing-individuals-with-ands-between (although it can and is, of course, also simply be used as am actual act name).