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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