***** NOT A TEACHER *****
Hello, s123:
Two teachers have already given you the answers to your questions.
*****
If you do not want to be confused by too many ideas, please do not read this post.
Some people might interpret those sentences in this way:
Mona: How is your sister Heba?
James: Oh, Maria is fine.
Mona: I did not ask about your sister Maria. I don't care about her. I wanted to know about your sister Heba.
James: Sorry! My hearing aid was not working. Heba is doing well, thank you.
In other words, if one has two or more sisters, some people feel that NO comma should be used.
Mona: How is your sister, Heba?
James: What did you say? I didn't hear you because a noisy airplane just flew over.
Mona: How is your sister?
James:Oh, she is doing fine.
In other words, some people feel that the comma indicates James has only one sister. Mona decided to say her name in the first sentence, but she left it out in the third sentence. Since James has only one sister, he -- of course -- knows her name anyway.
Compare:
"Their son Tony is a doctor, and their son George is a journalist." (The name "Tony" is absolutely necessary in order to identify the son who is a doctor. It is Tony, not George.)
"Their son, Tony, is a doctor." (They have only one son. The word "Tony" is extra information that is NOT necessary to the basic meaning of the sentence, so the commas indicate that "Tony" could be deleted or not used in that sentence. The basic sentence is: "Their son is a doctor.")
James