If a proper plural noun already ends in an -s, you have the ability to choose how possession is shown. Add an apostrophe after the -s. For example: The reunion of the Davis family = The Davis’ Family Reunion Or, add an apostrophe -s at the end. For example: The reunion of the David family = The Davis’s Family Reunion

Plural nouns refer to more than one person or thing, for example 'brothers'. To show possession when the noun is plural and already ends in 's', you just add an apostrophe to the end of the word. You don't need to add an extra 's'. For example: The brothers' feet were muddy. The cats' fur is very fluffy.