Is this an error?
When asked to translate: He is my brother, not my father, the answer they want is: C'est mon frère, pas mon père. It would not accept Il est mon frère, pas mon père. -- isn't "il est" appropriate here as well? (I did click the "still think you're right" button)
It's not really a negatives question, but it came up in the negatives section and I'm not sure how to categorize it better.
This one comes up a lot in questions, and I wish Duolingo would start to set out some of the basic rules, or at least make some of the insights "sticky". It's not really evident why one or other is used. So to answer your question ... .
It's not correct to say: "Il est mon frère ..." although as a non-native speaker it's hardly a gross error.
There is a good explanation of when to use "c'est" and when to use "il est" here: http://french.about.com/library/weekly/aa032500.htm . This one falls under the modified noun rule in the right hand column ("mon" being the noun modifier). And if you click through to the detailed rules on using "c'est" you will even see the example: "Tu vois cet homme-là ? C'est mon père."
It's annoying, this particular construction, because it's never properly addressed.
There are some vague insights on here, but not in enough detail.
In my French class, we were introduced to both forms, but NEVER told when to use which.