The general rule is that we would use "The X brothers" or "The X sisters".
And indeed, we might indeed use that form if we were talking about the two Grimms other than as named authors: "The Grimm brothers were both professors at the University of Göttingen".
It's not unheard of to use the form "The Brothers X" if we have cause to use it as a name, treating them as a single unit, but that's an unusual case to begin with, and an unusual form even then.
Still, the case does come up in terms of their co-authored books, in that we are using it as a single name for that authorship. The fact that it means we have a direct translation of the name they used in the original versions, "Die Brüder Grimm" adds weight to that particular choice.
It would still be unusual, but the decision made by the translator, that became the name stamped on the books, and hence the unusual generally became the particular here.
Likewise, while "The Brothers Karamazov" is an unusual wording compared to "The Karamazov Brothers" when the first translator decided that it sounded better to favour the wording that most directly corresponded with "Бра́тья Карама́зовы" the unusual generally became the particular there.
It's also worth noting that the choice wouldn't have been quite as unusual in the 19th century than it is now, again in contexts where we want to have a name for such brothers (or sisters, or family to consider how "Der Schweizerische Robinson" was translated) that treated them as a single unit—still less common than the other way around, but not quite as strange as it is now.
But the published books with "Brothers Karamazov" and "Brothers Grimm" on the front of them remained as they are as "Brothers X" became even less common, and stay crystallised in that form as we pick them up today, and so the Brothers Grimm remain the Brothers Grimm.