About Relative Pronouns
I have a sister who is smart.
I have a sister, who is smart .
I have a sister; who is smart .
Theses three sentences are all available?
And they are also same means?

Question

Answer

Trey is right insofar as the clause "who is smart" is non-parenthetical. In other words, the clause is needed to understand the sentence when a speaker has more than one sister. If the speaker has only one sister, then the clause is parenthetical, and a comma is needed.

Answer

The correct way to say this is: 
I have a sister who is smart.

"Who is smart" is an adjective clause and does not need to be preceded by a comma.   :)

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