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The definite articles meaning "the" are le, la, l' and les. The INdefinite articles, meaning "a, an, one" are un, une and the plural "some" = des.
When you say J'ai un le frère it will mean "I have one, the brother." Since your English says "and" here's the sentence:
J'ai un frère et il a vingt-sept ans.
This is a good review for you of both the definite & indefinite articles!