Is Brazilians proper possessive

No, the Word Brazilians is the plural noun:

• The Brazilians scored the winning play.

The Word Brazilian's is the singular possessive noun:

• The Brazilian's horse is a champion.

The Word Brazilians' is the plural possessive noun:

• The Brazilians' representative wishes to speak to the committee.

A proper noun and a proper adjective (Brazilian pottery) is always capitalized.