In aggregate I use the pluralizing apostrophe, but usually I'm talking about one specific story or another, in which case it is a tale from the Brothers Grimm. From, not of.
The titles of the earliest English editions, as far as I've been able to find out by browsing online library catalogues (hey - it's Saturday night!), use phrases such as "from the collection of", or "from oral tradition". That's in the 1820s, and the pair are referred to as "MM. Grimm". (Why French rather than German, I'm not sure.) Around mid-century the phrase "Brothers Grimm" appears, but in the 1890s we start getting books with "Grimm's" in the title, almost without exception in the singular, even when the subtitle makes it clear that there are two of them. E.g. in 1890: Grimm's Kinder- und Hausmärchen. Being a selection of the choicest fairy tales by the Brothers Grimm. With explanatory notes by W. J. Hickie. In that case, Jacob is listed as the "main author" in the catalogue. It's odd.
No. In the first case, the s marks the genitive singular. In the second case the s marks the nominative plural and the apostrophe is a sign indicated there would be a genitive s, if there wasn't already a plural in s. The apostrophe certainly never marks a plural per se. It is used to differentiate the genitive from the plural in s.
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Grimms' = possessive, a thing belonging to two, plural Grimms. Location of the apostrophe produces the plural.
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