Here you have modified noun "frère", so you must use "c'est", more:
I guess I'm fuzzy on what counts as modified. In the link they give the example of "He's a lawyer," which they translate as both "Il est avocat" (unmodified) and "C'est un avocat" (modified). Does "un" really count as a modifier? Isn't it just an article? What about "le"? Would you say "Il est l'avocat" or "C'est l'avocat"?
Any articles or possessive adjectives are modifiers: un, une, le, ma, mes…
Use "C'est …" when it is followed by a noun with a modifier: C'est un avocat.
Use "Il est …" when it is followed by an adjective or a noun without a modifier: Il est avocat. Il est bon.
Now, when to use which?
As a (very simplified) rule of thumb: Use "C'est an avocat" speaking about a person present. Use "Il est avocat" speaking about a person absent.
It is explained in the lightbulb notes at the beginning of this skill level and the preceding one. Here is a good article: